Abstract: Based on the study of thin section, cathodoluminescence, scanning electron microscope, fluid inclusion and X-ray diffraction data, the authors quantitatively analyzed the porosity evolution of tight sandstone reservoir in Huagang Formation in Structure Y, Xihu sag. It reveals that secondary pores are the primary. Compaction effect is the dominant factor that decreased the original porosity by 34.6%, and makes the most contribution in the early diagenetic stage. Cementation occurs in the whole diagenetic stage, and decreased the original porosity by 3.3%. Carbonate cementation is the main cementation, followed are silica cementation and clay mineral cementation. The dissolution which occurred in the late diagenetic period increased the porosity by 7.1%.