Abstract: For the study of gas-liquid two-phase transient flow characteristics with the power shutting down transition process in the reactor coolant pump, with the help of CFD, the gas-liquid two-phase flow variation and radial force were researched by experimental verification at different gas contents and the calculation results were verified by experimental test. The results show that the numerical simulation data agree well with the test data and vortex near the back of the impeller is produced, which makes the gas-phase region become large and the corresponding gas volume fraction increases. The capacity of the impeller to convert energy reduces with the reductions of flow rate and rotation speed, which leads to a large number of gas-phase in the impeller and guide vanes. When the gas content is small, the gas volume fraction will reaches a maximum before it declines in rotating dispersed form. When gas content is large, the gas volume fraction in the impeller increases with the reduction of flow rate. Gas content has a great influence on speed in flow passage of the impeller, especially for that close to blade inlet. The unbalance of radial force begins to fall off with the increase of gas content, and its maximum unbalanced radial forces changes from imbalance region between positive and negative value to that of negative value prevailing.