With the building of some large-scale water control project in the western region, it is needed to fill the high earth-rock cofferdam in the deepwater, the design and construction of the cutoff wall is important to the safe operation of the earth-rock cofferdam. Based on the constituve model of Duncan E-? and combining with the project, this paper analyzes the law of the stress and deformation of the earth-rock cofferdam by adopting the method of the nonlinear finite element. According to the construction process and different affecting factors, the paper also studies workability changes of the plastic concrete cutoff wall in the process of cofferdam construction by comparing a variety of project. It demonstrates the rationality of the design of the cofferdam, and provides certain reference to the safe construction and operation of the cofferdam.