In order to study the dynamic closing process of submerged flat gates under the effect of fluid-solid coupling, a flat gate at the intake of a hydropower station was selected as the research object, and a detailed fluid region and gate model was established. RNG k-ε turbulence model, VOF (volume of fluid) model and dynamic mesh technology in ANSYS Fluent were adopted for the numerical simulation analyses. The simulation results were then compared with the physical test data to analyze the stress and deformation of the flat gate. Compared with the pressure and flow field distribution at the measurement points in the physical model test, the simulation results are accurate and reliable. The maximum equivalent stress on the panel increases gradually during the gate closing process, which is significant at the upper flange of the main crossbeam close to the middle of the gate, but relatively small at the lower flange due to the fact that it is almost unaffected by the water pressure. The overall maximum displacement of the flat gate is concentrated at the bottom flange, and gradually increases with the gate closing process.