You're confusing a economic system for a political system. China is indeed communist because there is only one political party. China is also capitalist because

You're confusing a economic system for a political system. China is indeed communist because there is only one political party. China is also capitalist because it uses most capitalist tenets (things sort of mingle where politics and economics meet). On the other hand the old Soviet Union was a communist political system with a command-economy. The state decided what was built, where you worked, and how much you earned. China hasn't been a true marxist-communist since the late 70s. When Deng Xiaoping turned the economy (only the economy) towards capitalism.