The Dayan method in Mathematical Treatise in Nine Sections, which is also called Chinese remainder theorem, is a great accomplishment. Its origin and development, however, are not known clearly. According to Qin Jiushao’s preface, scholars have argued that Dayan comes from the Fangcheng of astronomers, which is not a real Fangcheng in Nine Chapters on the Mathematical Art. This article holds that astronomers’ Fangcheng is a real Facngcheng while its purpose is different. The article also presents the original meaning of Yuejifuyueou, which refers to odd number and even number. The fact is that Dayanqiuyi rule can be easily viewed by deducing astronomers’ Fangcheng， which not only tells us what is Qin’s contribution to the Dayan method，but also offers a new idea about its origin and development. Using the Fangcheng of astronomers, a series of convergent fractions can be naturally obtained. And Zu Chongzhi’s Circular Constant and other convergent fractions in history of China can be explained anew.