Ordos Basin is the typical inland depression lake basin of Mesozoic. By using the methods of the diagenetic fluid inclusion of reservoirs, authigenic illite dating and bitumen with different periods, analysis of the petroleum accumulation period, it is considered that the formation of Mesozoic oil reservoir is oil migration and accumulation. The formation rule for low permeability lithologic reservoir of Mesozoic in Ordos Basin has been summarized. Under the effect of abnormal high pressure which was generated by low compaction and hydrocarbon-generating pressurization of Chang 7 source rocks, hydrocarbon migrated up and down by overlying high permeability sandstone, formed large-scale lithologic reservoir in Chang 4+5, Chang 6 and Chang 8, and by the passage system of micro fracture and anterior Jurassic paleo-river sand, formed structural lithologic reservoir in Chang 2 and Jurassic reservoir.