In the early 90s of the last century, after the collapse of the Soviet Union, the 3,300-kilometer-long border at the western end of China and the Soviet Union became the border between China and Kazakhstan, Kyrgyzstan and Tajikistan. The border negotiations between China and the Soviet Union on this section began in the 60s, and now they have become negotiations between China and the three Central Asian countries, and the situation is even more complicated. 
Among them, Kazakhstan and Tajikistan reached border agreements with China in 1998 and 1999 respectively, and each returned part of the territory occupied by Tsarist Russia to China. Here I will mainly talk about the territorial dispute between Kyrgyzstan and China. Let's start with a brief review of the region's history. The three Central Asian countries all belonged to the vast Western Regions in Chinese history. Before the Han Dynasty, the Kyrgyz region was called "Qikun" or "Jiankun". During the reign of Emperor Wu of the Han Dynasty, most of the territory of Kyrgyzstan was included in the sphere of influence of the Han Dynasty. Later, the Han Dynasty established the Western Regions Protectorate to govern 36 countries, and at its peak it expanded to 50 countries. During the more than 400 years from the end of the Eastern Han Dynasty to the two Jin and Northern and Southern Dynasties, the Central Plains was in turmoil and had no time to take care of the Western Regions. It was not until the tenth year of the reign of Emperor Taizong of the Tang Dynasty (640) that the Tang Dynasty established the Anxi Protectorate and regained control of the Western Regions, including Kyrgyzstan. With the decline and demise of the Tang Dynasty, Kyrgyzstan changed hands several times over the centuries. 
During the Yuan Dynasty, Kyrgyzstan belonged to the Chagatai Khanate of Genghis Khan's family. By the time of the Qing Dynasty, most of the eastern and southern parts of Kyrgyzstan were included in Xinjiang, and the western territory belonged to the Kokand Khanate, one of the Qing vassal states. In the third year of Tongzhi of the Qing Dynasty (1864), the Qing Dynasty was forced to sign the "Sino-Russian Treaty on the Exploration and Demarcation of the Northwest Boundary" with the Tsar**, ceding about 440,000 square kilometers of land in western Xinjiang to **. It contains most of Kyrgyzstan. Later, Tsarist Russia annexed the Kokand Khanate and took possession of all of Kyrgyzstan. In 1926, Kyrgyzstan changed from a state to an autonomous republic, and later joined the Soviet Union. In August 1991, Kyrgyzstan officially seceded from the Soviet Union and became a sovereign state. 
The territory of today's Kyrgyzstan covers an area of almost 200,000 square kilometers. Due to the complicated history of modern times, there are several disputed territories between China and Kyrgyzstan, which have been resolved in the past two or three decades. The 1,096-kilometre-long border between China and Kyrgyzstan stretches from Khan Tengri in the north to the Zaalaysky Mountains in the south, and there are four major disputed areas, namely in the Khan Tengger Peak, the Yingyan River area, the source of the Qiongwusan Kush River and the Maritabar River. In January 1992, China and Kyrgyzstan established diplomatic relations. In May of the same year, Kyrgyzstan and China issued a joint communiqué acknowledging the results of the border negotiations between the Soviet Union and China from 1987 to 1991, and were willing to gradually resolve the remaining border issues with the Chinese side on this basis. This saves a lot of time for negotiations. 
Even so, direct negotiations between China and Kyrgyzstan have been going on for seven years. The first issue to be resolved is the Anglo-Border disputed area. The Yingyan area is a basin named after the river that flows along the Ying. On the map of China, the national boundary is on the ridge to the north of the basin; On the map of the former Soviet Union, the state border is biased towards the ridge to the south, which creates a 200-square-kilometer disputed area. Due to the difficulty of access to the basin from the Kyrgyz direction, the Chinese side gained an advantage in the dispute. In 1996, the two sides confirmed that the British region belongs to China. Negotiations in 1998 resolved two border issues, namely the disputed area of Khan Tengri and the Maritabar River. As one of the main peaks of the Tianshan Mountains, Khan Tengger Peak is 6,995 meters above sea level and is located at the junction of China, Kazakhstan and Kyrgyzstan, with a disputed area of 130 square kilometers. 
In the "Supplementary Agreement on the Border between China, Kazakhstan and Kyrgyzstan" signed in 1998, China, Kazakhstan and Kyrgyzstan returned the disputed area of Khan Tengri to China. The 220-square-kilometer Maritabal River dispute area was transferred to Kyrgyzstan in the Sino-Kyrgyz negotiations in the same year. The most arduous negotiation between China and Kyrgyzstan was the disputed area of the source of the Qiongwusan Kush River. According to the agreement between the Qing Dynasty and ** after the demarcation, the two sides should be bounded by the watershed in the north, and the disputed area should belong to China in its entirety. However, when the Soviet Union negotiated with China, it did not recognize the original agreement, and the disputed area of 2,844 square kilometers was formed with the main ridge of the South Tien Shan Mountains as the boundary, which was under the actual control of the Soviet Union. 
In the Soviet map of the 80s, the disputed area of the Qiongwusan Kush River was all within the territory of the Soviet Union. On the map of China at the same time, the region was all within China. Therefore, the negotiations between China and Kyrgyzstan on the region have become difficult. The 1996 China-Kyrgyzstan Border Treaty failed to resolve the issue. By 1999, after many negotiations, China and Kyrgyzstan had signed a supplementary agreement to the boundary treaty. The two sides reached an agreement on the issue of dividing the disputed area of the source of the Qiongwusan Kush River. Of the total area of 2,844 square kilometers, Kyrgyzstan received 1,984 square kilometers and returned 860 square kilometers to China. That is, China took back 30% of the disputed territory. Given the complicated history of the region, this result is actually quite good for the Chinese side, after all, the relations between China and Kyrgyzstan have always been friendly, and Kyrgyzstan has also shown great sincerity in the negotiations. In 2004, the Sino-Kyrgyzstan border demarcation work was completed, and the two sides signed a final protocol on the demarcation of the national border, and the territorial dispute was completely resolved.