Abstract: D flow over spillway is a critical flow with free surface. Due to the typical nonlinear nature of the problem, even though on the hypothesis of ideal potential flow, it is very difficult to be solved by use of numerical method,an efficient potential flow solution has not been found. An orthogonal transformation algorithm for 3-D flow is developed in this paper. This algorithm makes it possible to apply the free surface displacement method originally proposed by the first author, to iterate simultaneously unknown free surface position and energy head(or discharge). Computed test shows that the numerical method for 3-D flow over spillway is successful,and a good agreement between the computation results and experiment data can be achieved.