Both the Fengtai and Fangshan seismic stations are located on the Babaoshan fault in the southwest of Beijing, the two stations’  three-dimensional deformation observation curves have annual change, but their annual change law are opposite with each other. The results show that: the above mentioned change of the curves, may be associated with the fact that two stations are respectively located on the internal and external branch of Babaoshan fault , for the internal branch is reverse fault, while the external branch is normal fault, so the phenomenon may be caused by the different nature of the internal and external branch of the fault .