Abstract The CAD method is applied in the hydraulic design for fire pump, based on the factory’s producing practice. We build the mathematic model of the impeller’s axis plane projection, and parameterize the design of axis plane projection, and formulize the area checking of flow channel. Hence, the computer can automatically design axis plane projection and check the flow channel’s area. The paper gives the changing rules of the ρ-L curve and its optimizing methods. The CAD design method can greatly improve the precision and speed of the hydraulic design of fire pump’s impeller.