Abstract: Blade number, as an important structural parameter for centrifugal pump impeller, directly affects both external performance of the impeller and internal flow field. We create two impellers with 5 and 6 blades respectively using CFD software , and analyze the internal and external mechanical property of the impellers under the same operating condition. Through the comparison of H~Q, η~Q characteristic curves, the internal pressure and relative velocity of the two impellers, the influence of different blade number on impeller performance and the causes are analyzed. The comparative and analytic results can help design professionals to deeply understand the flow mechanism of impellers with different blade number and correctly select the blade number in design.