Carbonate cements are the most common diagenetic mineral in the sandstone of the 4th Member of the Xujiahe Formation (T3x4) in western Sichuan basin. The carbon and oxygen isotopes of different types of carbonate cements were analyzed using laser micro- sampling, in the T3x4 in the study area. Combined with petrographic characteristics and fluid inclusion analysis, the formation periods and genetic mechanism of different types of carbonate cements have been identified. Carbonate cements developed in the sandstone of T3x4 in three phases. The δ13CPDB values of the calcite in Phase Ⅰ are -0. 13‰~1. 53‰, while the δ18OPDB values are -9. 92‰~-6. 41‰. The δ13CPDB values of the calcite in Phase Ⅱ are -2. 44‰~1. 85‰, and the δ18OPDB values are -13. 97‰~-10. 06‰. The δ13CPDB value of the calcite in Phase Ⅲ is -5. 34‰~0. 75‰, and the δ18OPDB value is -16. 55‰~-14. 44‰. The δ13CPDB values of dolomite in Phase Ⅲ are -0. 97‰~-0. 81‰, and the δ18OPDB values are -13. 32‰~-12. 91‰. The carbon and oxygen isotopes of carbonate cements gradually became lighter from early to late, indicating that the precipitation and temperature increased, and the influence of organic fluids becomes stronger. The Phase I carbonate cement formed at the end of the Late Triassic (in the shallow burial stageof the reservoir) was directly precipitated from the supersaturated alkaline pore fluid derived from sedimentary water, and the source of “C” mainly comes from dissolved CO32- in sedimentary water. The Phase Ⅱ carbonate cement precipitated in the Early Jurassic (temperature about 45~70℃). Its formation is slightly affected by organic fluid. Its “C” sources include (1) light “C” from organic fluids, (2) “C” form original pore water, and (3) heavy “C” released by weak dissolution. The Phase Ⅲ carbonate cement formed in the Late Jurassic (temperature about 100~140℃), which is closely related to the injection of organic acid fluid. The main “C” sources of the Phase Ⅲ calcite are organic light “C” and part of the heavy “C” released by dissolution. The origin of the Phase Ⅲ dolomite is obviously controlled by the dissolution of carbonate rock debris in sandstone. The main “C” source is the heavy “C” form dissolution.