In order to maximize the benefits of cascade reservoirs in the lower reaches of Jinsha River under different typical years and different operation periods, the optimal operation model of reservoir groups in the lower reaches of Jinsha River was designed to maximum power generation and minimum square sum of water shortage. NSGA-II method was used to solve it, which obtained the non-dominated solution set under different typical years and partial periods. The replacement ratio was introduced to analyze the competition between the two targets. In addition, the water level process under different preferences in dry years was analyzed. The results show that between power generation and water supply targets,the competition is mainly reflected in the reservoir impounding period and the water supply period. The coordination is between the flood season and the pre-flood ebb period. The replacement ratio of water-power generation target shows an increasing trend with power generation. According to the replacement ratio curve, the influence on water supply is obtained with increasing unit generating capacity under different cascade generating capacity. Setting reasonable generating capacity in actual dispatching can maximize the target benefit of power generation and water supply.