Abstract: Mencius' political ethics world takes the kingly way as its fundamental purpose. His doctrine of the goodness of human nature and his advocacy of the kingly way together constitute the whole of the ethical world. When discussing the way of benevolence and righteousness, Mencius emphasized that the "benevolent and righteous behavior" of the sage king Shun originated from "understanding the common things and observing human relations", from which we can see that understanding things and observing human relations are two aspects of the construction of the sage king's personality and the development of political practice activities. The ultimate relationship between understanding things and observing human relations also constitutes a fundamental problem consciousness in the annotations and interpretations of Mencius in successive dynasties. Under the perspective of Mencius' philosophy, the ontological aspect of understanding things and the ethical practice of observing human relations constitute the two sides of "benevolent and righteous behavior" in a way of beginning and end, pointing to the way of "benevolent and righteous behavior" that combines the outside and the inside and the typical ideal personality type of Confucianism, and then presenting the internal unity between Confucian theory of mind and the construction of political order. The political ethics of Confucianism is the study of the world.