The invention discloses a method for designing a centrifugal impeller splitter blade. The method includes the steps that a middle cross section, a front cover plate cross section and a rear cover plate cross section of an impeller are determined; three split flow points on the three cross sections are determined on the three cross sections respectively according to the principle that flows on the two sides of each split flow point are equal, and three bone lines on the three cross sections are obtained; and the three bone lines are mixed and scanned into a curved surface, the curved surface extends towards a front cover plate and a rear cover plate of the impeller till the curved surface intersects with the front cover plate and the rear cover plate, a work face of the impeller splitter blade is obtained, and the thickness of the work face is increased to obtain the back side of the splitter blade. The designed splitter blade can equally divide water flow in a flow channel between two adjacent long blades into two parts equal in flow, speed distribution at an outlet of the impeller can be effectively improved, hydraulic loss in the impeller is reduced, and the lift and the efficiency of the centrifugal impeller are improved.