Abstract: The water environmental impact assessment(EIA) index of thermal power plants within a regional power net is the basic of processing power plant regional plan EIA.Three regional water EIA indexes of thermal power plants were put forward through research on the common characters of river basin environmental impact from thermal power plants.Various index weightings were calculated through the Sum/Product Method and the comprehensive water EIA index provided based on various weightings.Assessment indexes were used as a case in regional water EIA incurred by power plants within East China power net.The result shows that impact degree of regional water environment caused by power plants within East China power net from large toward little in order is the Huangpu River,the Taihu Lake,the Chaohu Lake,the Huaihe River,the Yangtze River and the Qiantang River.