This study investigated flood propagation and diffusion mechanisms in the Three Gorges Reservoir area. A one-dimensional hydrodynamic model was developed using the Runge-Kutta temporal discretization method and Discontinuous Galerkin spatial discretization, applied to the reach from Cuntan to Three Gorges Dam. The model validation utilized a flood event during the 2018 July to August flood season, specifically simulating the peak discharge of 42900 m3/s recorded at Cuntan Station on August 5. Results indicate a total flood propagation duration of approximately 20 hours from Cuntan to the dam, with the Cuntan to Wanxian segment (307 km) requiring 11 hours and the Wanxian to dam segment (288 km) completing in 9 hours. Quantitative analysis of spatial discharge gradients and velocity variations revealed flood wave diffusion coefficients exceeding 363 m2/s throughout the study area. Specifically, weaker diffusion capacity was identified in the Baishatuo to Zhongxian reach (average coefficient 3421 m2/s), while stronger diffusion occurred in the Zhongxian to Wanxian and Fengjie to Wushan reaches (average coefficient 8426 m2/s).