流域上游建库蓄水改变了水库下游断面洪水的情势，必须定量地评价水库对下游断面防洪形势的影响。建立了水库对下游断面的防洪作用评价模型，以清江流域为例，分析水布垭水库对隔河岩断面的防洪作用。研究结果表明：水布垭水库和区间均发生千年一遇洪水，隔河岩断面洪峰流量在无水布垭水库时为22,880 m3/s，有水布垭水库时为21,570 m3/s，洪水重现期从1000年减小为550年。水布垭水库和区间均发生二百年一遇洪水，经水布垭水库调蓄后，相当于天然情况下水布垭水库和区间均发生百年一遇洪水。水布垭水库的修建提高了下游隔河岩断面的防洪能力。The hydrological regime of the natural flow at the downstream site has been changed after the reservoir was built. The quantitative evaluation of reservoir’s flood prevention function for the downstream site is very important and an assessment method is established. The Shuibuya reservoir in the Qingjiang basin was chosen as the case study. The flood prevention function at the Geheyan site with or without reservoir was compared and analyzed. If 1000-year flood both occurred at Shuibuya reservoir and interval basin, the peak discharges at Geheyan site were equal to 22,880 m3/s and 21,570 m3/s without and with Shuibuya reservoir respectively, and the return period was reduced to 550 years. If 200-year flood both occurred at Shuibuya reservoir and interval basin, it was equivalent to 100-year natural flood condition after reservoir storage regulation. This study indicates that the flood protection standard of the Geheyan site is greatly improved when the Shuibuya reservoir has been built.