Sandstone reservoirs mainly develop in the second member of Shahejie Formation（Es2）in Zhenglizhuang oilfield of Boxing subsag，Dongying sag. Now the reservoir evolution is not clear because of its large vertical change. To solve this problem，core and thin section observation，data from vitrinite reflectance，and grain size of the study area were semi-quantitatively analyzed to determine the impact of reservoir diagenesis on porosity evolution. The study result indicates that the reservoir space of the target stratum is mainly composed of primary intergranular pore and intragranular dissolved pore.And the reservoir porosity evolution is mainly influenced by mechanical compaction，cementation and dissolution，among which the mechanical compaction and cementation reduce the porosity seriously. Compaction decreased the original porosity by 19.8% averagely and cementation decreased the original porosity by 14.1% averagely，which suggests that the original porosity is reduced more seriously by compaction than by cementation，and that compaction plays an important role in the whole process of porosity evolution. Dissolution is an important diagenesis process that improves reservoir pore space and mainly occurred in middle diagenetic stage A. Eventually，dissolution increased the porosity by 2.6%.