It is not entirely correct to say that the CCP, the Chinese Communist Party, is the only legal party in the People's Republic of China.
At least two other political parties, the San Min Chui(Three Princples of the People), and Young China are both legal and have representatives in the National People's Congress and other legislative bodies.
As was the case in most Eastern European Communist states (but not the Soviet Union), parties other than the Communist Party are legal to the extent that they accept the "leading role of the people's party" and enter into the National Front.