First of, it's important to note that Stalin was playing his own games and he didn't wholeheartedly support the Chinese communists during the civil war. However, once the war was won he supported China with technical assistance, military aid and capital loans.

The conflict between these emerged from ideological conflict and that China coveted lands in inner Asia that had been Russian for the Tsarist age. Mao was also displeased with Khrushchev's politics, and Chinese Maoists called him a revisionist for steering the U.S.S.R. away from Stalinist practices. In 1962 Mao accused Khrushchev of pusillanimous behavior during the Cuba Missile crisis. China were opposed to the Soviet-backed intervention by Vietnam into Cambodia. In 1972, Russias maritime rights in the Pacific and border territory in Manchuria caused both nations to continuously confront each other with large armies in other areas. At the same time, China's relations with the USA steadily improved.

Source: Palmer, Cotton, Kramer: A History of the Modern World