The Changxindian formation which outcrops on the Southwestern Beijing suburb is the deposits of Early Cenzoic. The statistics of the rock composition of Changxindian formation indicated that Jurassic extrusive rocks takes the predominant part in the lithologic composition while materials from the Sinian Sub-group come next, the intrusive rock components are insignificant.Basing on its special lithological composition and the features of geologic structure of the source area, the authors try to reconstruct the evolution of the landforms of the Xishan (West Mountain region).1. During the deposition of Changxindian formation the folded topography of Yan-shan period in Xishan Mountain region was peneplaned, then it was uplifted in the Himalayan movement, consequently, an inversion of relief has occurred.2. Guangting Gorge of Yongding River has been formed after Eocene.3.Dashi River and Guangting Gorge belonged to two different drainage systems in Eocene, the development of karst around the Dashi River region appeared earlier than that of the Guangting Gorge region.