Abstract: The data of acoustic travel time from 151 prospecting wells in the lower assemblage of Yanchang formation in western Ordos basin are used to analyze the abnormal pressure distribution in it by equilibrium depth method in order to comprehensively discuss the relationship between the abnormal pressure and the reservoir distribution as well as its hydrocarbon migration and accumulation discovered in the studied area. The study shows that the abnormal pressure occurs in Chang?7 and Chang?8 oil layers in this area. The abnormal pressureof Chang?7 is generally higher than that of Chang?8 by 8~12 MPa, so the difference acts as the principal dynamic power to make the hydrocarbon migration downward. Under such a pressure difference setting, most of hydrocarbon reservoirs have been discovered in the areas with high abnormal pressure difference