Reason:

(1) In the late 1960s and early 1970s, there was a serious centrifugal tendency in eastern European countries, which greatly weakened the control of the Soviet Union. In particular, in 1979, the Soviet Union invaded Afghanistan and was unanimously condemned by the East and the West, leaving the Soviet Union in unprecedented diplomatic isolation.

(2) After decades of infighting in the Cold War of the last century, both the United States and the Soviet Union (especially the Soviet Union) paid huge economic, political and military costs, which made the Soviet Union realize that there was no real final winner in the Cold War. In addition, China and the Soviet Union are both major countries of the Eastern Group, and making bad friends with China will only be detrimental to the Soviet Union.

(3) From the perspective of international political structure, especially after the end of the Cold War, the world peace movement has made new progress and the trend of globalization has become increasingly obvious, which has also created favorable conditions for the relaxation of Sino-Soviet relations.

(4) The three major obstacles between China and the Soviet Union have been eliminated. During the hostile period of Sino-Soviet relations, from the standpoint of China, three preconditions must be met to ease the relationship between the two sides, namely: 1, the Soviet troops withdraw from the Sino-Soviet border and outer Mongolia; 2. Urge Vietnam to withdraw its troops from Cambodia; 3. Soviet troops withdrew from Afghanistan. At first, the Soviet Union tried to avoid these problems, but with the development of the times and the change of the situation, the Soviet Union gradually changed its position, which also provided good conditions for the warming of Sino-Soviet relations.