In an intake and discharge channel of an electric power plant or a LNG plant, surging phenomena of water may cause a siphon break or an overflow at pits, and then may give a damage to the facility. A new numerical simulation method to predict surging was developed. In the method, the “slot model” was applied to the pressurized flow, and the MacCormack's scheme is used to simulate the mixed flow condition of sub-critical and supercritical flow. The water level in a discharge basin which is usually set at the end of the discharge channel can be calculated simultaneously. The laboratory tests were carried out to verify the simulation model for three types of channels. Three types of channels were a pressurized channel, an open channel and a channel which had a vertical bend at the end. All the channels have a discharge basin at the end. It is shown that the model can simulate an unsteady surging phenomenon for almost all types of channels.