On the basis of hydrologic data at upstream and downstream stations of Three Gorges Reservoir(TGR) before and after the impoundment, significance test was performed on the trend of longterm flow variation observed at Cuntan station, Wulong station, and Yichang station by TFPW-MK method. The impact of  TGR  impoundment on runoff process was also comparatively analyzed.  Results revealed that no evident impact on the runoff process was found, while the yearly runoff process was somehow influenced. The discharge volume from December to next June was larger than natural runoff, while was reduced remarkably from September to November with a maximum decrease of 12 015 m³/s in 2009. Having been optimized by the impoundment scheme at the end of flood season, the maximum decrease of discharge volume fell to 7 923 m³/s.