Based on core observation, cast thin sections, scanning electron microscopy and physical properties, combined with well logging interpretation, this paper studied the reservoir characteristics of Chang 8 oil reservoir set in Zhenyuan area, Ordos Basin. The result shows that the reservoir suffered acidic alteration with secondary pores developed. The reservoir space is dominated by remaining primary intergranular pores, intergranular dissolved pores and microcracks. Intercrystalline pores between clay minerals have also a little contribution. The reservoirs are controlled by sedimentary facies, detrital composition, diagenesis and tectonism. The favorable microfacies controlled the spatial distribution of the reservoirs. The clastic composition and grain size are directly related to physical properties. The diagenesis has both good and bad effects. The compaction and cementation made the majority of primary pores lost. The dissolution formed secondary pores which greatly improved the porosity and permeability, and they are crucial to the high-quality reservoirs.