According to main factors which affect the design of a composite impeller with ultra-low Unit speed centrifugal pump—the leaf number of the composite impeller’s short blade design, the relative position of the blade’s radial inlet, the bias degree of the blade’s circumferential direction, the deflection angle and so on. The paper applies Orthogonal method and establishes 16 design schemes. The preferentialness design of Composite Impeller’s short blade with ultra-low unit speed centrifugal pump was gained by numerical simulation. Contrast to the method of permutation and combination, this kind of method can decrease the designer’s workload greatly. As the results show, the composite impeller’s inner stress field and velocity field distribute reasonably, H - Q curve method is smoother, curve directs to large discharge and the work performance becomes well by applying orthogonal method.