In the thousands of years of development and evolution of ancient China, sometimes it split, sometimes it unified, but after each division, there would be greater unification, which is the law of division and unification in ancient Chinese history. Why does this law come into being? The study found that there are two fundamental reasons: one is all major ethnic groups in China have common blood and ethnic origin. Second, based on this same root and homology, China complex and China identity effect were formed. The analysis of a large number of prehistoric myths and legends of Zhuang, Yao, Hun, Xianbei and many other ethnic groups, as well as the rich historical documents of the Central Plains Empire, shows that many ethnic groups that had appeared in history have consanguinity and cultural homology with the Central Plains, which was regarded as the core area of Chinese orthodox culture. Just because of the limitation of historical conditions, it was inconvenient for them to communicate with the Central Plains. As time goes on, the two sides did not understand enough and the development level was different, which gradually widened the gap.