According to the lithologic combination of the existed exploration wells, the changed laws of the well loggings, and moreover the integration of the basin development-evolution process, 3 sets of regional cap rocks and 10 sets of local cap rocks are discovered and recognized in Lower Cretaceous Beier Sag. The studies show that the regional cap rocks are characterized by wide distribution, big thickness, whereas the local cap rocks are small and thin. The sealing abilities of each set of cap rock are mainly determined by pressure and diagenesis degree of the cap rocks distribution and moreover the the thickness, continuity, expulsion damaged faults etc. By means of choosing the following evaluating indexes such as sedimentary stage, monolayer thickness and cumulative thickness, damaged degree by the faults, degree of the rocks by the cycle, diagenesis evolution mud-strata ratio, expulsion pressure and so on, the sealing abilities of each set of cap rocks in the sag are comprehensively evaluated. The re- sults show that the abilities of the cap rocks in Beixi Subsag and Beizhong Subsag are relatively better in a whole, on the contrary, the abilities in the middle and southern parts of Beizhong Fault-uplift Belt are comparatively theworst; the middle-good and Sqd; and moreover cap rocks in Sqtb and sealing degrees are mainly presented for the three sets of regional cap rocks in Sqd1^1, Sqd1^2 the local cap rocks in Sqn1 and Sqn2; the middle-bad degrees are shown for the the local cap rocks in Sqtb and Sqd2.