Abstract: Based on the observation under microscope and the analysis on the data of SEM, electronic probe, isotopes of carbon and oxygen in carbonate cements, and combined with the burial history and structural evolution history, the paper identifies the diagenesis and reservoir space types and establishes diagenetic evolution sequences for different types of reservoirs of the Middle Jurassic Shaximiao formation in the western Sichuan depression. The study shows that different reservoirs have different accumulation characteristics and diagenetic evolutions. Compaction acted as the main destructive effect for the pores in far-from-provenance, feldspar-rich reservoirs whose reservoir spaces were dominated by feldspar-dissolved secondary pores and the reservoir densification time was late; compaction and cementation acted as the main destructive effects for the pores in near-provenance, debris-rich reservoirs whose reservoir spaces were dominated by residual intergranular pores and the densification time of this kind of reservoir was relatively early. The analysis on phase, occurrence and homogeneous temperature of inclusion indicates that the Middle Jurassic gas reservoirs are featured with multiple periods of hydrocarbon charging. The coupling relationship between reservoir densification and hydrocarbon accumulation reflects that the far-from-provenance, feldspar-rich reservoirs have a feature of hydrocarbon accumulation and reservoir forming simultaneously and the near-provenance, debris-rich reservoirs is characterized by densification before reservoir forming