The Xianbei ethnic group originated in the northeast and gradually became stronger in the northern part of China. From the late 3rd century CE to the 6th century CE， they established ten local regimes in northern China. Among them， the most powerful ones were the Northern Wei （386—534）， Eastern Wei （524—550）， Western Wei （535—556）， Northern Qi （550—581） and Northern Zhou （557—581）， which constituted the Northern Dynasties in Chinese history. Under the influence of Han culture， the Xianbei ethnic group of the Northern Dynasties actively and proactively integrated themselves into the Chinese nation community. The development of mathematical astronomy in the Northern Dynasties was particularly remarkable. This article traces the development process of the mathematical astronomy studies in the Northern Dynasties，and observes the process of the Xianbei ethnic group integrating into the Chinese nation community by analyzing the historical evolution of the Northern Dynasties' calendar system. The following understandings are， therefore， formed： The Xianbei ethnic group directly targeted the fundamental issues of the Chinese mathematical astronomy culture， actively accepted， inherited， and developed it. The Xianbei and Han scholars jointly studied and promoted the development of mathematical astronomy， reflecting the unity of the Xianbei and Han people in one big family. The Northern Dynasties widely absorbed the mathematical astronomy achievements of the Southern Dynasties， and both of them jointly inherited and continued the traditional Chinese mathematical astronomy culture. The development of mathematical astronomy in the Northern Dynasties shows a clear process of ethnic integration.