Both China and the USSR, along with most of the east Bloc, attempted economic reforms in the 1980s. For a variety of reasons China's was a lot more successful than that of the USSR. While this played a part in the survival of the Communist regime in China, perhaps more importantly was the relative lack of political reform in China and the survival of a group of revolutionary elders in that country.
In 1989 China Tienanmen Square and 1991 Soviet August Coup the hardliner Communists in each country were faced with protesters in their capitals, in both cases the party first secretaries were reform sympathizers who could not be counted on to preserve Communism and were deposed, in both cases an attempt was made to bring in the army to suppress the protests.
In the USSR, the hardliners deposed Gorbachev because they believed that he was destroying the union. However the hardliners backed down when faced with popular protests, largely because they did not have the determination to and questionable capacity of shedding the blood of Moscow citizens to preserve the regime: certain Russian generals had sided against the hardliners. This in turn was largely because there exist alternative centers of power to the central Soviet government due to political reforms, notably that of Russian Republic president Boris Yeltsin who led the opposition to the coup. With the failure of the coup the hardliners were out of power permanently and Yeltsin was able to dismantle the Soviet Union completely in what amounted to a power grab from Gorbachev.
In 1989 China however, the alternative center of power to the formal party structure were a clique of revolutionary elders (known as the 8 immortals) who had the final say over all major decisions made in the country. When the Politburo dithered on the response to the protests and party leader Zhao Ziyang refused to to suppress the protests, Deng and the rest of the elders overrode him, backed the hardliners, and gave approval for the tanks to be sent in. The immense prestige and influence of the elders were instrumental in enabling the Communist state to crush dissidents and ensure the survival of the regime.
Tienanmen itself was largely caused by the negative effects of economic reforms, notably inflationary pressure caused by price liberalization and the growing corruption within the government. What saved the Chinese Communist government was the determination of a powerful ruling clique within the party capable of enforcing unity as well as the lack of any political alternatives dissidents could turn to. What destroyed the Soviet Union was that, by 1991, the political process of reform had shattered the hardliner's capacity to enforce their will across the country and to turn the clock back to 1985. While there is little doubt that on the long run, the economic success in China is a linchpin of the survival of the Communist party, and that it can be said that the economic failures of Communism was what started Soviet political reforms in the first place, the key factor behind the survival of the PRC and not the USSR was due to the political structure and political personal in power at the time each state faced their crisis with a protesting mob.
I wouldn't put it down solely to economic reforms. In fact, as mentioned in the other comment, Gorbachev initiated some radical reforms of his own.
He introduced the concept of Glasnost (political openness) and Perestroika (restructuring of the economy). In a sense, it seems similar to what China did (more capitalistic market and allowing certain protests).
However, Gorbachev's plan was meant for a more gradual time scale, as the Soviet Union still maintained control over huge sections of the economy like prices and production, leaving a rather small and limited role for foreign investment. (1)
On the other hand, China released its iron grip on its economy, setting up its Special Economic Zones (Shenzhen etc), to attract foreign investors. They also had a reformed national bank policy providing funds/capital for many state owned organizations unlike their Soviet counterparts. (2)
Other than being too little and too late, it also probably due to the lack of understanding how the market would change according to the political climate for the Soviet Union. These policies had little time to take effect before the people lost faith in Gorbachev (3).
It is also worth noting that China maintained a much tighter grip on its political scene and people, which in way facilitated its transition to a capitalist economy.
Another major event occurring was was the numerous Soviet bloc countries declaring their independence/turning away from communism, a problem which China never had, functioning as the catalyst of the USSR's demise.
Edited: Added source