Based on the analytical data of cores, logging, common thin section, casting thin section, scanning electron microscope and physical properties, the author studied the reservoir characteristics of Pinghu Formation sandstones in the K structure of Xihu Sag. The results show that the Pinghu Formation sandstones have experienced multiple stages of diagenesis, mainly including compaction (pressolution), cementation, dissolution and reached the late stage of diagenesis. Reservoir pore space includes mainly primary residual intergranular pore and feldspar dissolved pore, and the secondary dissolution is common. Sedimentary environment is the basic factor affecting the reservoir properties. Compaction (pressolution) is the main cause of reservoir densification. The quartz overgrowth and carbonate cementation make the reservoir space further compressed. The development of secondary pores and the abnormal high pressure are the main reasons for the improvement of the physical properties of the deep reservoirs.