To solve the problem of anti-seepage effect of earth-rock cofferdam in the navigation-power junction project,this article takes Longxikou Navigation-power Junction project in Minjiang River as engineering background,and adds a sub cofferdam difference on the inner side of the cofferdam to solve the problem of cofferdam seepage.The article studies the advantages of sub cofferdam in quality,construction period,and cost by comparing the reinforcement of high-pressure jet grouting piles and water pump drainage.The practical application shows that the sub cofferdam can effectively enhance the anti-seepage of the original cofferdam.Compared with the scheme of using high-pressure jet grouting piles or water pump forced drainage on the basis of the original cofferdam,the sub cofferdam is a good choice in terms of construction difficulty and economic benefits.The research results can provide reference for similar projects in the upper reaches of the Yangtze River.