This article aims to review the interpretation on the astronomical knowledge in terms of Yŏk-hak易學(the Studies related to Yijing易經) articulated by the Confucians of the Chosŏn朝鮮 dynasty through the study of Sŏ Myŏng-ŭng徐命膺(1716-1787)'s works, and to discuss what significance Sŏ's interpretation on astronomical knowledge in terms of Yŏk-hak had in the process of Chosŏn Confucian's receiving the Western astronomy in the 18th century, which started from the 17th century. For this purpose, I had analyzed Sŏn-ch'ŏn-sa-yŏn先天四演, Pi-rye-jun揆禮準, and Sŏn-gu-je先句芹, which show Sŏ's astronomical ideas. As many Confucians of the Later Chosŏn dynasty had thought of astronomy as associated with Yŏk-hak, they tended to associate the astronomical knowledge with the philosophical thoughts of their own. This tendency had it's origin in two major traditions: The one is Sang-su-hak象數學 of the Han漢 dynasty, through which Yŏk-hak began to be related with the knowledge of Ch'ŏn-mun天文 and of U-ju宇宙, the other is Do-sŏ-yŏk-hak圖書易學 or Ŭi-ri-yŏk-hak義理易學 of the Song末 dynasty which constitutes one of the major components of Neoconfucianism with speculations on the T'ae-gŭk太極. Sŏ's interpretation on astronomical knowledge represents these traditions revealed in the 18th century Chosŏn. In Sŏn-ch'ŏn-sa-yŏn, Pi-rye-jun, and Sŏn-gu-je, Sŏ had interpreted most of the astronomical knowledge in terms of Yŏk-hak, including Western astronomical knowledge. For example, he supported Chi-won-sŏl地圓說(the idea of the roundness of the earth) by using Ha-do河圖 and Nae-do內圖 of Sŏn-ch'ŏn-bang-won-do先天方圓圖, and he also interpreted Dong-sŭng-ji-ch'a同升之差, belonging to the specific knowledge of Yŏk-pŏp暦法, as the arrangement of Ship-i-byŏk-kwae十二辟卦 or P'al-sun-kwae八純卦 represented in the Sŏn-ch'ŏn-bang-won-do. Moreover, he proposed new Kwae-gi-sŏl卦氣說 which assigned some Kwae卦 of the Sŏn-ch'ŏn-bang-won-do to the weather of the each region of the earthly surface according to the latitude. It seems that Sŏ's Yŏk-hak-wise-interpretation intended to reduce the antipathy against the Western civilization, which prevailed over the intellectual environment of the Chosŏn dynasty in the 18th century. This antipathy stemmed from Chon-ju-ron尊周論, together with Hwa-i-ron華夷論. Sŏ tried to makes it easy to receive the Western astronomical knowledge by his interpretation. For this purpose, he used the "theory of Saint-origin of the astronomy" as his principal argument, which insists that Ch'ŏn-mun and Yŏk-pŏp had been created by Saint Fu Hsi伏羲. It was the "theory of Saint-origin of the Western astronomy" that Sŏ applied such theory to the Western astronomy transferred from the 17th century. I think that it was for the intellectual justification that he had tried to elaborate the interpretation on the Western astronomy in terms of Sang-su-hak象數學, and that it was for the historical justification that he adopted the "theory of Saint-origin of the Western astronomy". We can see his efforts of the above-mentioned intellectual justification in his remark that the Ch'e-yong體用 relation is coming into existence between Sŏn-ch'ŏn-yŏk先天易 and Western astronomy.