The Minjiang Qianwei navigation-power junction adopts the construction diversion scheme of three phases and six sections to ensure continuous navigation during the construction period. In view of the problems of tight schedule and safe flood control risk in the third phase of the project，we propose to use the left lock wall to block the water and control the flood control during the diversion construction of the second phase cofferdam，and to demolish part of the second phase longitudinal cofferdam in advance under dry land condition. The results show that in the construction process of large ship lock in mountainous rivers，the use of the gate wall to hold back water and control the flood season can reduce the amount of underwater weir removal，improve the quality of longitudinal cofferdam removal，shorten the demolition period of the cofferdam，which has the significant economic and social benefits.