Underground gas storages （UGS） are widely used to store the excess of produced natural gas during periods of low demand, and relieve the pressure during periods of high demand. Depleted natural gas reservoir is generally the best choice against others. Proper selection of a depleted gas reservoir is essential to the succession, steady and reliability of UGS in the long run. Sealing capacity of caprock is an important index for measuring the performance of gas storage reservoir. In order to analysis feasibility of UGS in JB area, the development characteristics of caprock were studied, the sealing gas effectiveness of caprock was evaluated. it was concluded that the main caprock was the formation of evaporative carbonate platform, and the breakthrough pressure of cap rock was high. The cap rocks with good sealing capacity were distributed in the area between S224 well and S39 well. The sealing ability was comprehensively evaluated by use of the development features and sealing capacity of caprock.