Abstract: In tailrace tunnels of some hydropower stations, a transient Mixed Free-Surface-Pressure Flow (MFSPF) may occur, so a global computer model that can detect and simulate the mixed flow is needed. In this paper, a new MFSPF computation method (Characteristic Implicit Method) with good stability based on implicit finite difference scheme has been offered. Transient simulation programs have been developed based on the new method and other computation models for large-scale hydropower stations with MFSPF. By use of these programs, transient simulations of four large-scale Hydropower Stations such as the Three Gorges and the Xiluodu are studied. And the pressures in tailrace tunnel with MFSPF and other flow are analyzed and compared. Suggestions are made for the optimum design of tailrace tunnels with MFSPF.