Abstract: Based on the analysis of thin-section, scanning electron microscopic observation, vitrinite reflectance, X-diffraction and physical quality, the paper studied in detail the diagenesis of Kongdian Formation in Dongying sag.It is indicated that the original clastic component and the early cementation generated by the sedimentary differentiation determined the prost diagenetic process.The typical diagenetic phenomenon includes the intensive compaction, anhydrite cementation, multiperiod carborate cementation, multiperiod clay minerals cementation, intensive siliceous cement, intensive interpenetration replacement between minerals, solution of calstic and cementation, which influences deeply the reservoir of the deep-seated clastic rocks.The conclusion has been verified that compaction was the main cause of high pyknosis of the deep-seated clastic rocks, another cause was the cementations of anhydrite, carborate and siliceous, solution of feldspar and carbonate cementation was the main diagentic cause of improved reservoir.