Abstract: Terrigenous sediment rock develops with a great thickness in the foreland basin of the south part of Longmenshan Mountain, but the sandstone of which distributes most widely and its content ranks first. On the basis of observing and describing the macro characteristics of the sandstone in detail, the mineral composition and texture of the sandstone of Mesozoic and Cenozoic are studied by means of thin section and grain size analysis, the results show that both the composition maturity and texture maturity range from middle to low. The sandstone is classified, according to the scheme of genesis composition. In the light of the characteristics of sandstone as well as sedimentary features of deposited strata in the foreland basin, it is concluded that the provenance of the sandstone is Longmenshan orogenic belt, and the sedimentary model is established finally.