Why is China called a Republic?

It considered itself

Explanation:

This state considered itself to be the continuing sole legitimate ruler of all of China, referring to the communist government or "regime" as illegitimate, a so-called "People's Republic of China" (PRC) declared in Beijing (Peking) by Mao Zedong in 1949, as "mainland China", "Communist China, or "Red China".

China was see considered to be the continuing solo legitimate ruler of all China so that is why it is called republic