First, in this paper, a new type of interactive optimization method for multiobjective optimization problems is suggested. The method is made up with the calculation phase to generate a tradeoff curve from the Pareto optimal solution set and the decision phase to choose a preferable point on the curve. And these phases are alternated each other and a final preference solution is found eventually. Next, the method is applied to an optimal control problem of multipurpose dam where the storage water can be supplied for five use purposes, i.e. hydroglectric power generation, industrial, agricultural, municipal and adjustment water. The application results are compared with some specific cases and the effectiveness of the method is proved.