In order to design and select water sealing gasket for the joint of large open shield tunnel under high water pressure, the concept of effective contact stress is put forward. A complete numerical model of groove, rubber water stop tie and elastic sealing gasket of Nanjing Heyan Road Tunnel Project is built by software ABAQUS. The waterproof performance of the joints of segments under typical segment dislocation and opening conditions is analyzed, and the deformation and contact stress distribution characteristics of the sealing gasket of segment joints under the conditions of segment dislocation and large opening are revealed. The simulation results show that: (1) The distortion interaction between the gasket and gasket contact surface can improve the effective stress ratio on the contact surface. (2) The effective stress ratio between gasket and groove is lower than that between gasket and gasket in case of circular hole gasket, which is a key consideration in the waterproof design of gasket.