According to the formula of the ratio K of the maximal axle power to the rated axle power of the non-overload centrifugal pumps, K was expressed as the function of specific speed, impeller outlet angle and number of blades by calculating the formula. The three-dimensional image of K and contours about specific speed and impeller outlet angle of K under different numbers of blades were drawn based on the function. K is not close to the number of blades; while the impeller outlet angle and specific speed have a greater effect on K . Under the same K , the higher the specific speed was, the bigger the impeller outlet angle would be. Reasonable impeller outlet angle can be chosen according to the specific speed and K from the contours. The example showed that the method can make the design of non-overload centrifugal pumps successfully and quickly, and can avoid the blindness in choosing the impeller outlet angle with the traditional method.