Abstract: The double-cone seal structure may undergo leakage failures in high pressure vessels. Its sealing pressure and mechanism were investigated in this paper. The concept of inflexion pressure and the minimum sealing pressure were put forward. The main stud bolts forces in the pre-tightening and under the work condition were analyzed. The formula to calculate the radial deformation and sealing pressure of the double-cone ring were deduced, and then the inflexion pressure and the minimum sealing pressure were obtained. Compared with the FEA results, good agreements were observed. It is suggested the minimum sealing pressure be checked for GB150.