The initial water level of the Minjiang Laomukong navigation and hydro-power junction is 358 m in the preliminary design，and this paper objectively analyzes its shortcomings. Based on the principle of multi-objective decision，the optimization of the water storage level of cofferdam during the construction period for water retaining trial navigation and the initial power generation is studied，and a systematic simplified algorithm is proposed. The research conclusions obtained by the simplified algorithm are as follows：1） The optimized water storage level is 7.28 m lower than the preliminary design （one unit is used for power generation in the initial stage），and the power generation period is 21.6 d ahead of schedule，with significant relative benefits. 2） It can reduce the operation risk of the second section cofferdam in Phase II，save the total construction period and other spillover effects. 3）It also expands and obtains the results such as the optimal storage level when two units generate power in the initial stage.