Based on the single-stage self-priming centrifugal pump, a kind of self-priming device of multistage self-priming pump with high self-priming performance and external characteristic was proposed. Based on the positive and inverse diffuser, the backflow device was creatively designed and composed of gas-liquid separation chamber, outer shell, self-priming cover plate and gas-liquid mixture chamber. The highpressure water of finalstage pump chamber was sent to the first-stage pump chamber through the backwater device, and the gaswater was mixed repeatedly in the self-priming process. When the self-priming was completed, backflow valve in the self-priming cover plate was automatically shut off under huge pressure difference, which effectively improved the efficiency of self-priming pump. Based on the multi-objective fuzzy optimization design, the multi-objective optimization model with the maximum head of off dead point and minimum value of maximum shaft power was determined. Combined with design experience and technological needs, relevant constraints were established and solved through nonlinear extremum. Finally, the hydraulic optimization design of flow components was completed with the optimum solution. By doing the external experiments of multistage self-priming spray irrigation pump, it was concluded that the head of off dead point and maximum shaft power met the demand, which provided a new reference for optimization design of pump.