Xu Ai was the husband of Wang Yang Ming's sister. Xu Ai was also one of the earlist disciples of Wang Yang Ming so he very convinced in Wang Yang Ming's thought. However, because Xu Ai died at early age, his Neo-Confucianism thought did not develop into a mature system. Therefore, scholars mere consider that Xu Ai's thought was early representative of Wang Yang Ming's thought. In fact, there was the evolution that from Cheng-Chu Neo-Confucianism to Wang Yang Ming's Philosophy of Mind in Xu Ai's thought. Besides, Xu Ai expressed that he felt ridiculous when he learned Wang Yang Ming's thought in the beginning. Nevertheless, he learned Wang Yang Ming's thought more and more, he believed that Wang Yang Ming's thought is genuine successor of Confucius's thought. This shows Xu Ai's Neo-Confucianism thought was changed in deed. Consequently, this essay constitutes the system of "Mind is Principle "by the way of the evolution of Xu Ai's thought.