Based on the design of low specific speed submersible pump, the influence of impeller structure parameters on shaft power of pump was analyzed. Under the condition of the maximum value of shaft power being no more than the input power of matching electromotor, the method of reducing the maximum value of pump was further researched in order to reduce the power of the matching electromotor. Then the design discriminated formula of the structure parameters of impeller was presented. The value of shaft power was predicted by the Fluent software. The results indicated that the ratio of maximum power of pump to the power in designing condition was less than 1.2 when the presented discriminated formula was used in design. The results of the experimentation of prototype showed that every parameter met the rating requirements; the maximum value of shaft power of pump was reduced greatly. The results demonstrated the guiding role of the discriminated formula in practice.