The Late Jurassic volcanic rocks of the Lanqi Formation in the Beipiao area are composed of the lower andesitic breccia lava with tuffs and the upper trachyandesite. In this paper, the author thinks that andesitic magmas of Lanqi Formation are derived partial melting of foundered lower continental basaltic crust near the transition between the crust-mantle and occur the interaction of the mantle and crust, and trachyandesitic magmas from partial melting of foundered lower continental basaltic crust. The magma evolution of Lanqi Formation indicates that the process of the crust thinning from the thicken crust, it is an important significance to understand the deep process for the lithospheric thinning of the North China Craton during the Mesozoic era.