In numerical simulations of hydroturbine systems, the fluid in the draft tube keeps alternating between the free surface flow condition and the pressured flow condition. This combination of free surface flow model and pressured flow model has been a focus of researchers. Because of the huge difference in pressure wave speed, the two models differ much in terms of meshing even in the same method. To track the interaction point between free surface flow and pressured flow, a type of interaction point tracking method (IPTM) is proposed, which is based on the method of characteristics. Using a controllable iteration to solve the controlling equations of the boundary conditions, the concrete interaction point can be tracked accurately at each time-step. The method is validated in an actual turbine draft tube and compared with experimentally measured data. The results show that IPTM is equipped with high reliability and offers some advantages compared to the other existing methods.