The shale of Paleogene Shahejie Formation in Dongying Sag was studied，according to the data of core，thin section，scanning electron microscopy（SEM），electron probe micro element scanning and carbon-oxygen isopiestic，etc. The effect of diagenetic events on shale pore space was analyzed. The results show that：①The shale in Dongying Sag develops obvious laminated layers，and the mineral types in shale is complicated. The content of carbonate minerals，especially calcite，is generally high. Organic matter is abundant with medium vitrinite reflectance. The porosity of laminar argillaceous limestone is relatively high，and free oil mainly distributes in micro-pores and fractures. ②The major shale is at the middle stage of diagenesis，and 3 000-3 600 m is the key interval of pore development and evolution. The key diagenetic events during shale diagenesis include shrinkage and transformation of clay minerals，recrystallization of carbonate minerals，organic matter persistent hydrocarbon generation and acid dissolution. ③Recrystallization could form cleavage slits and intercrystalline pores，and the dissolution during the hydrocarbon generation and discharge process could generate internal dissolution pores in carbonate minerals，these pores form the effective pore space combination and increase the reservoir porosity. ④The inter-laminar cracks are not rigorous fractures，which is related to the seepage force induced by overpressure and high overpressure. Below 3 000 m，the fluid overpressure alone could not overcome the overburden pressure to open the crack. The inheritance and matching of diagenetic events such as acid dissolution，induced overpressure of hydrocarbon generation from organic matter and clay mineral syneresis，forming the pore spaces（combination）that is easier for fluid flow，thus becoming an effective seepage channel and oil pore space.