In the southwest area of China where earthquakes are frequent, the seismic excitation has a great impact on the safe, continuous and stable operation of radial steel gates. It is necessary to study the dynamic response of radial steel gates to the earthquakes and the effect of earthquakes on the safety of the gates. Here, a three-dimensional numerical model considering the coupling effect of water body and structure was constructed, and the seismic dynamic response of the gate was studied. According to the calculation results of the gravity dam-reservoir fluid-solid coupling model, this method was proved to be accurate and effective. Furthermore, the dynamic time history response of the gate was analyzed, and the hydrodynamic pressure of the gate was calculated and then compared with the standard method in the specification. The results show that the dynamic response analysis method proposed in this paper is suitable for the calculation of the seismic dynamic response of radial steel gates, the fluid-solid coupling effect has a great influence on the seismic response of radial steel gates. Considering the fluid-structure coupling effect, the calculation result of the dynamic response of the gate is significantly larger than that of the standard method in the specification, and the maximum value of the ratio is more than two times larger. The dynamic displacement response peak of the gate structure is at the center of the gate top panel, and the dynamic stress response peak occurs in the middle of the lower main beam. The dynamic calculation of fluid-solid coupling shows that the rigidity of the gate structure meets the specification requirements, but the strength does not. The structural dynamic response of the upper and lower main beams are weak, the seismic water pressure calculated by the standard high-head method is rather large, whereas the calculation result of the low-head hydrodynamic pressure of the surface hole is too small. The dynamic response analysis of the gate structure considering the fluid-solid coupling can reflect the true mechanical characteristics of the gate, and has guiding value for the structural design and safety evaluation of radial gates in the earthquake-prone areas.