Abstract: Based on the results of mecury injection and gas absorption analytical methods of caprock,the complete pore diameter distribution diagram of caprock and the capillary pressure curve can be obtained.The breakthrough pressure of a rock sample can be calculated directly by the simulated test of gas driving saturated rock water.Research shows that the breakthrough saturation values are different in different rock types.The values are from 19% to 37% with average value being 26% in mudstone,limestone,dolomite,siltymudstone,etc.,and 41% to 49% with average value being 44% in fine sandstone,siltystone,muddy-conglomeratic sandstone,etc.Combining the capillary pressure curve,the pore diameter distribution diagram and the lithology of a rock sample,a reasonable saturation value can be determined.The capillary pressure value corresponding to the saturation can be approximately taken as the breakthrough pressure of the rock sample under the fomation conditions.