The gist of it is that they were following the traditional notion of a flat square earth contained in a spherical heaven up until the introduction of Western astronomy in the 1800s.

The change in how they thought about it was largely brought on by the Jesuits who were active in China and in many cases served as astronomers to the court.

As a sice note, you might also be interested in this post from a few hours ago asking the same question about Japan. The answers are quite similar.