Based on the relationship between phase quantized levels in unit period and the smallest feature size of the Fresnel diffractive lens,a new optimal design method for diffractive element with large numerical aperture is proposed and the diffraction efficiency is discussed.The parameters of the multi level diffractive lens are determined.A design example of a Fresnel lens with large numerical aperture is presented,which is compared with the deep etching method and the phase matching method in the diffraction efficiency and in the number of etching.It was shown that the theoretical diffraction efficiency of the designed Fresnel lens is 55.4 % with the etching number of 3,which is competitive with the existing methods.