Abstract: Based on the analysis of development and microcosmic sealing characteristics of mudstone caprock reflected by the earth physical information of the lower part of Nantun Formation in Beier rift-subsidence basin,this paper chooses displacement pressure, thinckness of caprock, thinckness ratio of mudstone and stratum, monolayer thinckness of mudstone,abnormal pore liquid pressure and sedimentary environment as parameters to evaluate comprehensively the sealing abilities of mudstone caprock of the lower part of Nantun Formation in Beier rift-subsidence basin. It indicated that mudstone caprock of the lower part of Nantun Formation in Beier rift-subsidence basin developed well,and the diatropic continuity was good so that it can serves as caprock for the underlying oil reservoir. The caprock has the strong seal abilities,and in which the north and middle part have good or medium sealing abilities. Two conclusions are drawn:the first is the lower part of Nantun Formation mudstone caprock is one of the main controlling factors of the model of oil accumulation, where reservoirs accumulated in old buried hills and its oil mixed by many origions, the second is in Beier the distribution of oil in space was controlled by the mudstone caprock.