The genesis of medium-coarse crystalline dolomites of Middle Permian Qixia Formation in western Sichuan Basin is researched based on the geochemical methods and observation of outcrops, cores and thin-sections. The results showed that crystalline dolomites can be divided into three types: idiomorphic, hypidiomorphic, and xenomorphic. The residual bioclastic structure can be seen in moderately to coarsely crystalline dolomites. Fractures are well developed, and some crystalline dolomites are cut by stylolites. The pore space mainly consists of intercrystalline solution pores, solution vugs and intercrystalline pores. The solution pores are usually filled by idiomorphic dolostones and saddle dolostones. The δ13C values of the medium-coarse crystalline dolomites are in a range of marine carbonate rocks of the Permian strata, and the δ18O values in the north of Western Sichuan are higher than those in the south. 87Sr/86Sr values of dolomites in the north of Western Sichuan are also in a range of marine carbonate rocks of the Permian strata, and those in the south are higher than this range. The REE pattern model shows that the dolomites inherit characteristics of limestones. Ce of the dolomites shows positive anomalies, but Eu shows not obviously. La of the dolomites in the south of Western Sichuan shows relatively obvious negative anomalies. The study concluded that the grain beach face is positive for dolomitization, and the seawater dolomitization is the main genesis of medium-coarse crystalline dolomites during shallow burial stage, and at the late diagenetic stage they were suffering from hydrothermal modification in local areas. Seawater dolomitization in shallow burial stage is also the genesis of idiomorphic dolostones filled in solution vugs, but saddle dolostones are the products of later hydrotherm.