In this paper, a centrifugal pump of medium specific speed is taken as the research object, and the performance changes of the centrifugal pump are studied by setting five different ratio degrees of blades. The results show that due to the shortening of the blade length, the pump head and efficiency both decrease with the decrease of the ratio degree. In terms of static pressure distribution, the high pressure area in the volute expands from the outlet to the upstream with the decrease of the ratio degree, and forms an obvious boundary layer with the pressure in the impeller. Under the rated flow, the model pump with ratio degree R=2/3 has the best cavitation performance. When the number of impeller blades with a ratio of 2/3 was optimized, the flow field performance of the model pump with 8 blades was the best, and the head increased by 7.33% compared with the prototype pump at rated flow.