Abstract: This article is based on the topography, hydrology and sediment data measured by the Upper Yangtze River Waterway Bureau after the impoundment of the Three Gorges Reservoir to 175 m height to study the change of the streamwise backwater effect with the aid of the SMS of a planar 2D mathematical model. The change of the Longmenghao reach flow before and after the impoundment is analyzed, and the analysis results show that: (1) after the impoundment in non flood season, the river reach in the reservoir has the characteristics that the tail water is too high, flow velocity, water surface slope and water power have fallen sharply; and that in the flood season, the reach becomes natural just as that before the impoundment; (2) the average flow rate is taken to represent the average of the hydraulic factor condition; under the same flow before and after the impoundment, the Longmenhao reach flow simulation of different periods is compared, showing that the change amplitude of hydraulic factors in impoundment period is the largest, that in storage period is smaller, that in drawdown period is the smallest, and that in the flood season has no change; (3) weighted estimation shows that after the impoundment within the first year, the average stream power reduces by 47.9% compared with that before impoundment, and at the same time, the stream power yearly change distribution is extremely uneven, which will lead to big changes in sediment transport in the reach.