The Chongzhen lishu 崇禎曆書 introduced classical European astronomy, mathematics, and metrology to late-Ming China. One of the editors, Xu Guangqi 徐光啟 (1562–1633), studied “the science of du and shu” from translating Euclid’s Elements and applied it to calendar reform. This article discusses the meaning and peculiarity of “the science of du and shu” in Xu Guangqi’s thought. This article establishes three points. First, Xu criticized basic Chinese mathematical theory as unclear, since every dynasty inevitably amended the calendar once or several times, unlike the Western calendar, which was perfected by expansion and teaching by hundreds of generations. As a result, the “meaning and reason” 義理 was progressively better understood, and the “methods and numbers” 法數 became distinct. Moreover, the whole system went from the simple to the complex, which is one of the characteristic of axiomatic science. Second, the science of du and shu was introduced in the Dace 大測, Celiang quanyi 測量全義, Cetian yueshuo 測天約說, and Bili guijie 比例規解, which provided basic knowledge for calendar reform. Third, at the foundation of “the science of du and shu” was geometrical deduction and logic, its main application to astronomy being the establishment of the geometrical models of the heavenly bodies via observational data, demonstration of its model parameters, and empirical verification.