Aiming at the phenomenon of water fluctuation in the process of water pump start and accident pump stop in the high cistern of the circulating water system of thermal power plant. By using CFD software and VOF method, the transient boundary conditions were given by UDF. The water surface fluctuations of high cistern of pressure and open flow design a power plant were simulated during the simultaneous start of the three pumps and the simultaneous shutdown. The calculation results show that under the starting condition and accident condition, the bell shaped inlet channel with an open channel, which increases the free surface area of the bell shaped suction chamber, weakens the water disturbance at the high cistern than the channel pressure bearing elbow inlet channel. In the case of water fluctuation during the start of the pump and the shutdown of the accident, it is recommended to adopt the open circulation pump layout.