The main purpose of this thesis is to explore Zhu Xi’s exposition on the image of Mencius saint, and to analyze the connotation of the ancient title of “sub-sage”. This article analyzes Zhu Xi’s collections, quotations, and related documents, and finds that the ancients called Mencius the sub-sage, which meant that his talent was comparable to Confucius, not that he was the second sage. Since the Tang Dynasty, “sub-sage” mainly refers to Yanhui, and Mencius’s status is not as good as Yanhui. Although Zhu Xi had a great Confucian status as a respect for Mencius, he pointed out that Mencius was inferior to Confucius and Yan Hui through comparison. Zhu Xi explained Mencius’s disrespect of the emperor and Zhou Dynasty from the perspective of current situation. He tried to resolve the dispute of Mencius and helped to enhance the image of Mencius. However, Zhu Xi also faced the lack of Mencius’s remarks and established a more complete cultivation system through discussion. Through the research results of this article, everyone’s misunderstanding of “sub-sage” will be changed. And can have a deeper understanding of the image of Mencius and the development of controversial issues.