Abstract: By means of casting thin section, scanning electron microscope, cathodoluminescence, constant-rate mercury injection, fluid inclusions, X-ray diffraction, we studied the reservoir petrology, pore throat structure, types, and characteristics of diagenesis, and discussed the densification process and physical evolution of the reservoir of Pinghu Formation in Xihu Sag, East China Basin. Results show that the reservoir sandstone of the Pinghu Formation is mainly feldspar lithic quartz sandstone, the structure is mainly fine-medium grain with medium to good sorting. Intergranular pores formed the main reservoir space, and the pore radius ranged 130～190 μm, with throat radius of 0.2～10 μm. Burial compaction is the main cause of low permeability and density of PF reservoir. Depth, grain size, and argillaceous content determined the strength of compaction. In addition, the difference of secondary corrosion and cementation in later stage intensified the heterogeneity of reservoirs. In diagenetic stage, the scale of bio-dissolution in the Pinghu Formation reservoir was limited by fluid environment, and fine-grain particle deposition was not conducive to late dissolution, which are the two main reasons for reservoir densification. In diagenetic stage, the diagenetic environment was alkaline and gradually closed, after which a large number of iron-bearing carbonates, filamentous, and curved illite were built up and block the throat, resulting in the large-scale compaction of the reservoir.