In order to evaluate the tightness of water-sealed underground storage cavern, we carried out test on the efficiency of water curtain system. According to the pressure values of water boreholes at different hydrodynamic levels, we estimated the efficiency of water curtain system, and then by drilling new boreholes in low efficiency area, we could improve the connectivity and permeability of rock mass. Furthermore we present a method of calculating the key parameter, namely critical pressure value, and applied the method to calculate the critical pressure value of a water-sealed underground storage cavern in Yantai as an example. Test results proved that the method of calculating the critical pressure value is feasible. According to the calculated critical pressure value, we can accurately determine the rock area which needs more boreholes. Moreover, rock permeability is closely related with the efficiency of water curtain. In most cases, rock permeability ranging from 10^-7m/s to 10^-5m/s could meet the cavern’s tightness requirements. The process of obtaining the critical pressure value in this research is based on assumptions and needs further improvement in engineering practice.