Based on analyzing history, present, and future relationship of spatial interaction between Beijing and Tianjin, it is concluded that Beijing and Tianjin have been interdependent for quite a long time historically, and interaction strength between them increases annually and is developing towards integration direction since the era of Chinese economic reform and opening to the outside world. Its acing factors include distance, size, complemantarity, and intervening opportunities etc. According to the theory of spatial interaction, Beijing and Tianjin is adjoining and large, the radiation range is overlapping, and there exists strong complemantarity in natural resources, social and economic condition etc. Further, Intervening opportunities provide good conditions for the development of other provinces, cities, counties towns, and developing areas except Beijing-Tianjin economic region. Moreover, a new factor, institutions is introduced into the theory of spatial interaction, which possesses Chinese features. In order to provide suggestions for making policy in coordinate development of Beijing and Tianjin, its affection was also explored based on the history, present as well as future of spatial interaction between Beijing and Tianjin.