Based on the top seal of a large width-to-height flat gate, ANSYS software is applied to carry out finite element analyses to do some regular exploration research on the dynamic characteristics, and then further into the shape optimization design through harmonic response analyses, on the condition that sealing structure self-excited vibration occurs while the seal gets away from the gate panel. The results show that: the natural frequencies increase with the length of the seal and flow decreasing; tight force of the bolts on the sealing structure show stage influence on the natural frequency; it can improve stiffness of the seal structure while the pressure plate width increasing and the pore radius of the P-type head decreasing, which makes it take obvious advantage on overcoming resonance, fatigue,and other harmful results by forced vibration.