The “siyu 四餘” is an astronomical term designating the four invisible celestial bodies, Rahu (luohou 羅睺), Ketu (jidu 計都), Yuebo 月孛, and Ziqi 紫氣. The “siyu” was known to be related with the sun’s and moon’s motions, and the solar and lunar eclipses. Astrologically, it was called the “four hidden luminaries”(si yinyao 四隱曜) or the “eleven luminaries” (shiyi yao 十一曜) together with the seven luminaries (qiyao 七曜) of the sun, the moon and the five planets. In China, the “siyu” never played a major role in the official Chinese calendars, but it was widely embraced by the populace in association with astrology. From the Later Han period, the “siyu” was introduced to China from India and Central Asia, along with such Western religions as Buddhism, Manichaeism, and Nestorianism. They travelled to China across different countries and cultures in different periods. Consequently, several different versions of “siyu,” having different meanings and origins, were transmitted to China. In the process of transmission, moreover, their names and meanings underwent considerable change, and mixed with the Western and Chinese elements. This paper examines the complicated process of how the “siyu” was introduced to China, how its meaning was subsequently changed, and finally how it influenced Chinese astronomy and astrology.