Abstract: Exploration and development of Chang 63 reservoir of Yanchang formation in Huaqing oilfield, Ordos Basin has been challenged by strong heterogeneity and complicated microscopic pore structures. Based on the data of cast thin section, SEM, high-pressure mercury injection and constant-rate mercury injection experiments, the paper studies different diagenetic facies and the pore structures of Chang 63 reservoir in Huaqing oilfield. Moreover, logging responses of different diagenetic facies are summarized by combining with production test data. The results show that the throats of the reservoirs of chlorite cementation-intergranular pore facies and illite + chlorite cementation-intergranular pore facies are best developed; the pore throat radius of the reservoirs of illite cementation-intergranular pore facies and illite + chlorite cementation-corrosion facies are relatively small and the reservoir connectivity is poor to medium; the pore structures of tight reservoirs of illite cementation-corrosion facies, illite cementation facies and carbonate cementation facies are poorly developed. Different diagenetic facies have different microscopic pore structure characteristics. Throat radius is the major factor influencing reservoir flowing and storage capacity. The distribution of high quality diagenetic facies is influenced by sedimentary facies, diagenesis and microscopic pore throat characteristics