Abstract: During transient process of hydropower station, mixed flow of free surface and pressurized regimes usually occurs in the tailrace tunnel with sloping ceiling, and its flow patterns is relatively complicated owing to its features of three dimensionality. Accurate simulation of such free surface-pressurized flows is crucial to regulation guarantee parameters. This paper describes simulations of this mixed flow using 1D-3D coupling approach, and compares the results with those of virtual slot approach. We also discuss the difference of these two methods in modeling the interface of free surface and pressurized flow regimes and its oscillatory motion as well as their results of minimum hydrodynamic pressure in draft tube, demonstrating the flaws of virtual slot approach.