The invention discloses an impeller, a water pump and a design method of the water pump. The impeller comprises a front cover plate, a rear cover plate and vanes arranged between the front cover plate and the rear cover plate, wherein the radius of the front cover plate is larger than that of the rear cover plate, the edges of the vanes are located outside the edge of the rear cover plate, and the liquid flowing-out direction at a water outlet of the impeller is the axial direction. The water pump comprises the impeller, the impeller is arranged inside a pump body of the water pump, a pumping chamber cavity of a pumping chamber which is in the pump body and cooperates with the impeller to work is axially arranged, a separating tongue is arranged in the pumping chamber, the arrangement angle psi 0 of the separating tongue is from 15 degrees to 25 degrees, 1/6 part of the pumping chamber is located under the rear cover plate of the impeller, and the rest of the pumping chamber spirally and gradually exceeds the position under the rear cover plate. By means of the impeller, the water pump and the design method of the water pump, the liquid flowing-out direction of the liquid at the water outlet of the impeller is changed into the axial direction instead of the radial direction, and therefore the size of the water pump is decreased.