Abstract: It is proved by tests that the lower hydraulic efficiency (about 2%～8% lower than ordinary) is the main reason for the lower efficiency of non-overload centrifugal pump. By I Khalill' s numerical method, in hub-shroud plane using potential flow calculation to determine flow surface position and in blade-blade plane using completely viscous turbulence calculation, the inner flow of non-overload centrifugal impeller is calculated. The result shows that there exists neither flow separation nor apparent wake flow within an non-overload centrifugal impeller. Therefore, the lower hydraulic efficiency attributes to the larger hydraulic friction loss due to the larger blade wrapping angle and the narrow-long flow passage.