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The diagenesis and pore evolution process of tight sandstone reservoirs in the Upper Triassic Bagong Formation of the North Qiangtang Depression were studied by means of physical properties, casting thin slice, SEM, X-ray-diffraction and fluid inclusion temperature measurement. The compositional maturity and structural maturity of the Bagong sandstones are moderately low. Their physical properties are extremely tight. The main diagenetic effects include compaction, cementation, dissolution and metasomatism. The cement components are mainly siliceous, clay minerals and carbonate minerals, and the dissolution component is feldspar. According to the comprehensive analysis of various characteristics, the shallow buried area near the uplift is in the middle diagenesis stage B, and the deep buried area inside the depression is in the late diagenetic stage. Under the premise of considering the apparent volume change and compaction of rock, a quantitative calculation of pore evolution of different diagenetic facies is carried out. The calculation results show that compaction is the primary cause of reservoir porosity reduction, followed by cementation, while dissolution and the formation of chlorite lining are beneficial to reservoir porosity.