The purpose of this paper is to reveal the characteristics of Song Si-yeol's theory of Chunqiu, which inherited the great unity of the 'Gongyangzhuan'. At the time of Song Si-yeol, the Joseon Dynasty was humiliated by the Qing along with the collapse of the Ming Dynasty, and there were a lot of people who protested against the Neo-Confucianism after a war with the Qing. From this, Song Si-yeol declares that Joseon is civilized, that is, a civilized country in terms of region, tribe, and culture through the thought of great unity in 'Gongyangzhuan'. Since Qizi came to the east, Joseon has a close relationship with China in terms of tribe. In addition, Song Si-yeol asserts the great unity of the Confucian succession that leads to Confucius and Zhuzi through Jeong Mong-ju, based on the idea of respecting the Zhou. In terms of culture, this is to realize the great unity that even the Confucian scholars of Ming and Qing could not achieve it. From a regional point, it can be said that the conquest of the Qing Dynasty is an element of the great unity. In particular, Song Si-yeol pursues the conquest of the Qing Dynasty based on the politics of the true king. This is a different point of view from those who claimed the conquest of the Qing Dynasty by considering the logic or situation of the power at the time. And Song Si-yeol's theory of conquest of the Qing dynasty not only considers the lives of the people, but also seeks harmony between the ruler and the servants, and between the servants and the servants. Therefore, it can be said that Song Si-yeol's thought of Chunqiu is the idea of great unity that pursues the reconstruction of Joseon centered on the politics of the true king.