Abstract: The flow characteristics of stopping pump poses a significant influence to the natural circulation. In order to investigate the flow characteristics of centrifugal pump under small flow natural circulation condition, an experimental facility was designed, by which the pressure drop and loss coefficients of centrifugal pump were measured under forward and reverse condition. The experiment condition covers the Reynolds number range of 2.0×10⁴-1.5×10⁵. The experiment indicates that the reverse pressure drop is obviously higher than forward pressure drop under the identical Reynolds number condition. The loss coefficients remain almost unchanged when Reynolds number is higher than 8×104, however, the loss coefficients tend to increase with the decline of Reynolds number in low Reynolds number. The empirical correlations of loss coefficients are obtained based on the experiment. A CFD method was conducted to simulate the internal flow field of centrifugal pump. The calculation indicates that loss coefficients are predicted well with CFD. The RNG k-ε model predictions agree with the experimental values within 10%. The pressure drop is mainly concentrated on the interface areas of impeller and guide vane, etc. The flow field distribution between forward and reverse flow shows a significant difference. Local flows are more volatile in low Reynolds number.