Abstract: In order to break through the choke point of the traditional design of a telephoto lens,how to eliminate aberrance by means of hybrid refraction-diffraction was studied from the view of the phase polynomial of diffractive surface and phase transformation polynomial of general lens.The telephoto lens system consisted of only two pieces of optical elements made of FK1 glass has an equivalent optical performance to the traditional telephoto lens system made of five pieces of optical elements made of defferent kinds of glass.Which shows that the hybrid refractive-diffractive lens can effectively eliminate the aberrance of a telephoto lens with lower weight and simple structure.