Objectives: A major breakthrough has been made in the gas exploration on sucrosic dolomite reservoir of the middle Permian Chihsia(Qixia) Formation in the northwest Sichuan Basin. However, recent cores show that the development horizon and scale of the sucrosic dolomite is of great variability. Therefore, the key to the distribution prediction of this type of dolomite reservoir is to figure the genetic mechanism out. To solve the problem, study has been carried out on the Chihsia Formation in Shangsi section, such as petrological characteristics, carbon—oxygen isotopes and EPMA(electron probe micro analyzer) of the dolomite, to establish dolomitization model of Chihsia Formation. Methods:The dolomite development in field and petrological characteristics were studied through the outcrop observation, microscope analysis and Cathodo Luminescence observation. The paper combines the in- situ micro- electron probe analysis, isotope geochemistry characterization, petrological characteristics and geological background to trace dolomitization fluids. Results:Four characteristics of the sucrosic dolomite were summarized, including: massive dolomite, developing well in the middle of Chihsia Formation member 2, and leopard dolomite from dolomitic limestone, developing in the lower part of member 1, lower and upper part of member 2. ① Massive dolomite is characterized by medium—coarse crystalline and red luminous under CL(Cathode Luminescence), with Mg/Ca values 0.68, δ 13 C PDB and δ 18 O PDB average values 2.42‰ and -6.18‰ respectively; ② Leopard dolomite developing in the lower part of Chihsia Formation member 1 is characterized by powder—fine- crystalline, micro- sucrose texture and non- luminous under CL, with MgO/CaO values 0.54, δ 13 C PDB and δ 18 O PDB average values 2.42‰ and -6.18‰ respectively; ③ The upper part dolomite with medium—fine- crystalline structure shows a non- luminous core inside and a red luminous rim outside under CL, while the lower part dolomite with medium—coarse- crystalline structure shows dark red luminous under CL. Their MgO/CaO, δ 13 C PDB and δ 18 O PDB average values are 0.59 and 0.68, 2.16‰ and -6.50‰, 3.32‰ and -3.75‰ respectively. Conclusion:Based on the results from filed work and indoor materials arrangement, The results suggest that ① in the lower part of Chihsia Formation, there are two penecontemporaneous exposure surfaces, below which leopard porphyritic karst system developed. ② Eogenetic exposure surface has been identified on the top of Chihsia Formation with clear vertical zonation of karst developed below. Most of the karst dominant channels was dolomitized. ③ Both types of dolomites were formed by superposition of seepage—reflux dolomitization and hydrothermal dolomitization. Due to the vertical zonation and influence degree of karstification, there are differences between the result of two phases dolomitization.