Abstract: The leakage of toxic or combustible gases can lead to catastrophic consequences. The large-caliber ball valves are often used in mine ventilation or natural gas gathering and transportation pipelines. Sealing performance of ball valve is a key indicator of safe and stable operation of equipment. In this paper, the sealing performance of the sealing structure components of the 40-inch fully welded ball valve under the high-pressure and low-pressure sealing conditions is studied. The results show that in the case of forward and reverse sealing, the high stress region appears on the valve seat support ring, and the maximum equivalent stress is less than the yield strength. Besides, the high stress and large deformation of the valve seat appear at the two sharp corners in contact with the valve seat support ring. In high-pressure sealing, the contact pressure between the valve seat and the ball contact area is greater than the work pressure to ensure sealing. Compared with high-pressure sealing conditions, the contact area between the valve seat and the ball is significantly reduced during low-pressure sealing. It is necessary to ensure that the work pressure is greater than the design pressure, because too low work pressure will reduce the sealing performance of the ball valve. This article provides a research method for analyzing the two-way sealing performance of large-diameter ball valves, and can provide a reference for determining the sealing pressure range of large-diameter ball valves.