Abstract: In order to study the gas-liquid two-phase transient flow characteristics of the nuclear reactor coolant pump under the exhaust transit condition, the transit process of exhaust of nuclear reactor coolant pump was simulated by using CFX software and the heterogeneous turbulence flow model. By analyzing the pressure pulsations, vorticity and velocity distribution in the impeller and vanes flow channel, the flow variation law of exhaust transit process was obtained. The results show that the radial force of the impeller generates large fluctuations under gas-liquid two-phase flow condition, caused by the gas phase and liquid phase unevenly distributed in each channel of the impeller, and the slip effect between the two phases. It is easy to generate bubbles accumulating phenomenon within the channels of the twisted radial guide vanes on high inlet gas flow rate conditions. The phenomenon makes the flow area reduced, resulting in greater energy loss. The spherical symmetry structure of the nuclear reactor coolant pump volute makes the left side of the separation tongue appear low velocity area, plugging part of the outlet flow channel. It is also an important reason for the unstable operation during the process of the exhaust.