The reef and shoal facies reservoir in Changxing Formation and Feixianguan Formation are the important targets for natural gas exploration in Eastern Sichuan Basin. The lithology of the favorable reservoirs is dolomite, which are located on both sides of Kaijiang-Liangping trough. The Xinglongchang Area of East Sichuan Basin is located on the western side of Kaijiang-Liangping trough. The main gas producing formations are dolomite reservoir in Changxing Formation, which distribute in the high-energy reef and shoal facies zone prone to exposition. Since the control factors of the distribution of dolomite have been clarified, based on the analysis of rock structure, Mg/Ca ratio and the degree of order, combined with the geochemical characteristics, the genesis of dolomite in the Changxing Formation, Upper Permian in Xinglongchang Area of East Sichuan Basin is studied in detail. Results indicate that the dolomite is the product of replaced dolomitization during burial diagenesis period. The dolomitization fluid is probably the coeval condensed seawater trapped in the limestone and marlstone compelled by compaction. The conclusion comes from the following evidences: (1)The chemical composition of the euhedral dolomite developed along the suture line in the Feixianguan Formation is similar to that of the dolomite in the Changxing Formation, indicating that the two kinds of dolomites have the same origin and the dolomitization fluid may be the compaction fluid; (2)The δ13C of dolomite is 1‰~2.5‰(PDB) and the δ18O is -6‰~-4‰(PDB), which is accorded with the composition of δ13C(0~5.8‰, PDB) and δ18O(-7.5‰~-3.5‰, PDB) of the coeval paleo-seawater and the distribution of δ13C and δ18O of the typical burial dolomite supposing that the dolomitization fluid is probably the coeval seawater and the dolomite formed during burial; (3)87Sr/86Sr ratio of the dolomite (0.707108~0.707507) is under the changing range of paleo-seawater of the Late Permian(0.707108~0.707507) proving that the dolomitization fluid is coeval seawater; (4)The rare earth elements of dolomite represent the same characteristic with the seawater that the loss of LREE and the enrichment of HREE, which demonstrates that the dolomitization fluid is seawater derived; (5)The carbonate in Changxing Formation has the very low Fe concentration (<2000×10-6), but the dolomite has the higher Mn concentration(69×10-6~90×10-6, averages 76×10-6) and the brighter cathodoluminescence relative to the limestone, indicating that the dolomite formed under the reducing conditions during burial.