According to the regime of the Changjiang River, this paper sets up a numerical dynamics model of applying Preismann implicit difference method for difference treating of Saint Venant equations. Based on the best-imitating historical hydrologic data and recurring historical hydrologic process, each parameter in the differential equation is rated. The rated model ivS used to study the impacts of the Three Gorges Project on the discharge, in dry and rain seasons respectively, at certain stations along the Changjiang River, The results are as follows. (1) The impacts of the increased/decreased sluicing discharge from the Three Gorges Dam will reduce relatively with the distance of calculating section from Yichang Station after the Three Gorges Project operated. The relative reducing rate is about 0.5 during rain season, and about 0.67 during dry season in Datong section of the river. The reason is that the distance of calculating section from Yichang to Datong is not only longer, but also wider, and the capacity of river regulating is greater. (2) Because of greater roughness of the river basing during dry season(0.036 0 -0.036 5) than that of rain season(0.0260-0.0265), the damping effect of the river section during dry season is stronger than that of rain season, and the the impacts of the Three Gorges Project on discharge along its downflow of the Changjiang River during dry season are greater than that of rain season. The rate of the impact value between rain season and dry season is 1.0-0.75. when the sluicing discharge from the Three Gorges Project increases/decreases relatively after the Three Gorges Project operated.