If the underwater water retaining device is to be installed on the surface of the cylindrical structure，it is necessary to solve the two major problems of sealing and retaining water together with the cylinder structure and bearing the buoyancy of the device.Based on the analysis of existing water retaining structures，this paper puts forward a kind of underwater construction device subjected to top pressure of casing which is composed of water retaining structures and bearing structures.The water retaining structure is a split casing structure with a circular hole reserved at the bottom and an open top，and the water is retained by the close contact between the reserved circular hole and the cylindrical structure.The bearing structure is a well-shaped top pressure structure arranged on the top of the pile foundation，bearing the buoyancy on the water retaining structure and transmitting it to the two adjacent pile foundation structures.Combined with the installation practice of the floating mooring pile steel casing box of a high-piled wharf，the device design and the stability check calculation of pile foundation structures are carried out in this paper，and the construction key points and process control requirements are clarified.The device is reusable，economical，reasonable，and technically feasible，which provides a reference for the underwater construction of similar projects.