China wishes to retain power and to gain more. Taiwan having independence would undoubtedly lessen China's power and also significantly reduce its population. China has the greatest population in the world, and it wishes to have worldwide recognition and deference as a result, perhaps.
Hence its "one China" policy must not come under threat by Taiwan wishing to gain independence. The situation is affected by the ambiguity of the Taiwanese themselves, only some of whom feel Taiwanese rather than Chinese.
China does not let Taiwan declare independence because China is still hoping for reunification with Taiwan. Since the end of the Chinese civil war in 1949, the winning side established the People’s Republic of China (PRC) in Beijing. The losing side left for Taiwan and they have been separate ever since. China considers Taiwan a “wayward province”.
Also since being separate, the communists in China initially let the Chinese people suffer while the people in Taiwan have prospered so China also wants to continue the ongoing economic integration progress is has recently made in the world and with Taiwan.