Mingdao’s Theory of Nature (《明道论性说》) is Zhu Xi’s (朱子 1130-1200) special commentaries to Mingdao’s (Cheng Hao程颢 1032-1085) representative paragraphs on the theory of nature. From Zhu Xi’s commentaries to the theory of nature, it can be seen that he inherited and developed the theory, from “Theory of Nature” to “Theory of Mind and Nature.” Zhu Xi borrowed Mingdao’s argumentation method of comparing the “good” and “evil” of human nature to the “clear” and “turbid” of water. Based on the categories and allusions used in Mingdao’s argumentation, he specially added the categories of “heart” and “emotion,” taking the “good” as the starting point and ultimate goal, demonstrating that nature is good and returning to goodness through the psychological cultivation of “learning to overcome qi (difficult temperament) (学以胜气).” At the same time, Zhu Xi developed Mingdao’s “nature can be good” into “nature is good,” “teaching” into “learning,” and “self” into a sage. The key point is that Zhu Xi filled in the gaps in Mingdao’s theory of human nature, identified what is dubious, removed its contradictions, answered the question “while human nature is good, where evil comes from and where it goes,” and constructed his systematic theory of mind and nature.