Compaction, cementation, denudation and replacement are the main diagenesis types in the first member of Shahejie Formation in Qikou Sag. Denudation takes a significant part of improving the reservoir property when above 2 200 m. There are two secondary porosity zones developed in the vertical direction, with the depth respectively ranging from 2 700 m to 3 200 m and from 3 500 m to 4 200 m, and the peak value of porosity is at 3 150 m and 3 650 m respectively. Based on the data of vitrinite reflectance, ratio of clay mineral mixed layer and palaeogeothermal, the diagenetic stages of the first member of Shahejie Formation in Qikou Sag can be divided into 4 stages: early diagenesis A stage, early diagenesis B stage, middle diagenesis A1 substage and middle diagenesis A2 substage, and nowit is mainly in the middle diagenesis A1 substage tomiddle diagenesis A2 substage.