Question

how can we say that China is not a democratic country

Answer 1

China is not a democracy as it doesn't allow free and fair competition for power. Only one political party stands the chance to win the majority of seats and form the government and that party is the Communist Party of China.
Sure other opposition parties are allowed to stand for elections theoretically, but practically it doesn't not hapoen . This is so because the election candidates have to get the approval of the ruling CCP and it is only then that they can stand for a post. And the CCP is notorious for only approving candidacy of its own members or members of affiliated parties.

Answer 2

this is the reason why China is not called as democratic country