Abstract: In tailrace tunnels of large hydropower station, a transient mixed free-surface-pressure flow may occur, which has a complex effect on the stability and transient of the power plant. It is necessary to build a global computer model that can detect and simulate the mixed free-surface-pressure flow. In this paper, a new method, namely Characteristic Implicit Method, with good stability based on implicit finite difference scheme has been proposed. Compared with other methods, the new method resolves the calculation instability when the interface of the mixed free-surface-pressure flow passed the computational node. The normal forms of the hyperbolic system made up of the continuity and momentum equations are used and differenced in this method. Model experiment for tailrace tunnel of hydropower station has been conducted and the results of this method agree with the experiment very well.