The construction task of Qianwei navigation-power junction on Minjiang River is heavy and the time is short. In order to shorten the construction period，save the investment of cofferdam construction，and meet the overall schedule of the project and the navigation requirements during the construction period，it is necessary to remove the “second stage and second section cofferdams”，and use the gravity lock wall outside the ship lock to retain water during the flood season to continue construction. In view of the safety and stability of the structure when the external lock wall is used for water retaining in flood season，the paper puts forward the corresponding engineering measures by rechecking and calculating the structures of different water retaining parts，so as to meet the safety and stability of the structure，ensure the construction period of Qianwei ship lock，and save the project investment.