In order to rationally design the water seal of ship lift gate in the Three Gorges Project and reduce the cost of sealing experiment, the computer simulation design of air-pressure telescopic water seal for the ship lift gate of the Three Gorges Project was presented based on the basic principle of solid mechanics and nonlinear calculating theory. In the design, the tightness of cavity behind the air-pressure telescopic water seal and the expansive probability of seal head were also verified in the simulation. We obtained the required back pressures of sealing water under different initial clearance between seal head and sealing panel, as well as different working pressure of reservoir, providing data for the manufacturing and operation of seal device. The key technology during the design was the water-tightness simulating method of air-pressure telescopic water seal, and can effectively replace the traditional tightness experiment to greatly reduce the cost of design.