Abstract: Carbonate cement is the most important cement in the reservoir of the Middle Jurassic Shaximiao formation in the western Sichuan depression and is also one of the key factors of reservoir densification. Carbonate cement is very developed in the piedmont of Longmen mountain and its average content is about 12% in the area. Taking the sandstone of Shaximiao formation in the piedmont of Longmen mountain as the research object, the paper studies the formation mechanism of the carbonate cement based on the microscopic observation on samples and the analysis of carbon and oxygen isotopes in the carbonate cement and electronic probe. The study shows that the carbonate cement in Shaximiao formation in the area is dominated by intergrown and microcrystalline calcites with the δ13CPDB of -7.69‰~-2.13‰ and the δ18OPDB ranging from -15.34‰ to -9.60‰. The inorganic carbon acts as the main source of the carbonate cement. The cement formed at the temperature of 56.1~101.3 ℃, indicating an early diagenesis stage. Relatively low FeO and MnO contents in the intergrown calcites and relatively high FeO and MnO contents in a few microcrystalline calcites indicate that the carbonate cement was mainly formed at the early diagenesis stage. There are also some calcite cements developed at the late diagenesis stage whose formation is related to Fe and Mn rich fluids in the deep strata