CIA Factbook says they are 90+% atheist. http://www.cia.gov/cia/publications/factbook/geos/ch.html#People
####Daoist (Taoist), Buddhist, Muslim 1%-2%, Christian 3%-4% note: officially atheist (2002 est.)####
It says they are officially atheist, which is true. That means the government is atheist. There's no way of really determining the religiousness of the population since faith in God is generally suppressed. But my experience with Chinese college students is that they aren't atheistic at all.