Abstract: This paper presents a practical and effective 3 - D numerical model for spillway tunnel flow.In which potential now model is applied to the raised tunnel intake and a new method is developed for the free level section on base of the consideration of the influences of roughness and boundary layer development. The method is not only suitable to the internal boundary layer region but also to the external potential flow region. Thus it can reasonably simulate the flow characteristics of spillway tunnel. Results of some verified computations and engineering reveal that this numerical method has higher precision and can meet the requirements in practical engineering.