The deep water high earth-rock cofferdam in the 2nd phase construction of TGP is a hard and challenge project, whose degree of difficulty in the technological problems is much harder than that of the similar projects. During the period from 1998 to 2002, the cofferdam protected the construction pit of the left-bank dam and powerhouse, stood the test of the heavy flood in 1998, and made the 2nd stage construction completed smoothly. The quality of the cofferdam is good, and it fulfilled the historical tasks satisfactorily. The article introduced the whole course of the systematical research through the aspects of the experimental research, the design and construction, the organization and management, the safety monitor and the cofferdam demolition etc.