Abstract: Based on the analysis of casting lamella，scan electron microscope and X-ray diffractions of sandstone lamella ,this paper has conducted research on the petrology characteristic, diagenesis stage division and its influence on porosity of Chang-4+5 Reservoir in Yaodian oilfield of central Ordos Basin. It is indicated that the main reservoir sand in the area is arkose with diagenesis type including compaction, cementation and dissolution. Overall diagenesis process includes early stage (A and B phases) and late stage (A1 and A2 phases). The early compaction and multi-mineral cementation are the major factors that leads to poor petrophysical property of Chang-4+5 Reservoir, but the ehlorite film formed during the early stage helps to preserve some primary poriosity; dissolution at the phase A during the late stage, especially feldspar dissolution is one of significant factor controlling the quality of reservoir.