Abstract: Frequent extreme precipitation events have exacerbated the risk of flooding in the watershed; The current flood simulation methods face the challenge of insufficient accuracy in dealing with the risk of flash flood disasters, and there is an urgent need to develop high-precision hydrological models to improve the reliability of flood control decisions.The article constructs a one-dimensional and two-dimensional hydrodynamic coupling model for the Guobaoxi River Basin in Fujian Province, and systematically simulates the flood evolution process under different return period heavy rain scenarios, revealing the dynamic feedback mechanism between flood evolution process and terrain factors.By coupling one-dimensional and two-dimensional hydrodynamic model, the problems of mass imbalance and momentum distortion that may occur at the coupling interface using traditional simple interpolation or empirical methods have been fundamentally solved, thereby improving the integrated flood simulation system.Research has found that the maximum submerged area of a flood with a recurrence interval of 100 years is 198.62 km², which is 1.57 times larger than the submerged area of a flood with a recurrence interval of 10 years; There is a significant spatial coupling between the breach point of the embankment and the terrain depression area; There is a strong spatial correlation between flood risk distribution and geomorphic features.Model validation shows that the relative error of peak water level is less than 0.5%, and the prediction deviation of peak arrival time is controlled within 20 minutes. The research results provide quantitative technical support for optimizing flood control projects and emergency management in river basins, and have important application value for improving the flood control system in mountainous towns.