Abstract: Based on the mould thin section and thin section analysis of cores from the Fusan Member of 24 wells in the northern slope of the Qingtong Depression, the diagenetic characteristics, diagenetic phases and their relationship with the porosity and permeability of reservoir rock were studied. The results indicated that dissolution was the essential favorable diagenesis for the generation of secondary porosity, which was especially well deve loped in the interval of 2 350-2 880 m depth. And compaction and calcitic cementation were the main cause of decreasing porosity. In general, calcitic cementation was more severe in the west than in the east of the study area. In terms of diagenetic stages, there were clear differences in different areas: strong dissolution was occured in Mozhuang area; medium dissolution with strong calcitic cementation in some intervals was developed in Maoshan area; calcitic cementation was common in Yedian-Biancheng area; and South Dainan was characterized by strong compaction. In accordance with the diagenetic characteristics, the reservoirs in the study area were assessed.