Abstract: Mou Zongsan’s theory of unity consists of two sections: the early theory of perfection and the later theory of unity. Although the latter is more improved and at a higher spiritual level, it has more mistakes in some crucial theoretical issues, especially taking formlessness as a new a priori principle of unity and explaining formlessness with the concept of “putting down”. Because of these mistakes, Mou’s later theory of unity is much more confusing and complicated so that it is less accurate than his early theory of perfection.