Based on the comparison of different research methods on the hydraulic losses of suction boxes and discharge passages, the computational fluid dynamics method is adopted to simulate the internal flow of four different pumping systems and to investigate the influence of rotational speed of pumps on hydraulic losses of suction boxes and discharge passages. The computed results indicate that the internal flow of suction box is affected by the pre-swirl caused by impeller rotation, the value of hydraulic loss is smaller than that when the suction box is operated solely without pump. The hydraulic loss of suction box keeps unchanged at the same flow rate when the rotational speed of pump is changed. The hydraulic characteristics of discharge passage are influenced by the flow conditions and residual circulation of pump diffusers. The internal flow field is not conformed to the affinity law and the hydraulic loss of discharge passage is unequal at the same flow rate when the rotational speed of pump is changed. The validity and reliability of numerical computation are verified by model test results of pumping systems. 