The second line shiplock of Mengtli hydroproject in Beijiang River of Guangdong Province situates in a mixing zone with carbonaceous limestone,carbonaceous shale and siltstone,limestone interbedded and soft rock & intensive karst zone developed.In view of the stability of deep foundation pit,stability of excavated slope higher than 60 m,karst treatment，we propose reinforcement measures and construction techniques for the trench of the earth wall,and adopt a new type of wall cofferdam structure in the lower part of the wall and upper retaining wall,which effectively reduces the backfill,excavation and internal force of the construction platform,and meanwhile provides a reliable basis for optimizing the foundation pit construction plan and lays a solid foundation for ship lock construction.