Abstract: The diffractive optical element is one of the main solutions to achieve uniform illumination, which is achieved through the pure phase element. There are sub-diffraction spots because of the discontinuity of pure phase element, which makes the beam lose 15% or more energy. Thus a diffractive-refractive method based on the traditional input-output algorithm is developed to design the diffractive optical element. This method makes part of a diffractive optical element continuous in phase, which reduces the light intensity in the discontinuous region of the element, therefore reduces the intensity of sub-diffraction spots. Through studying the influence of spherical phase factor on output light intensity, the phase parameter can be optimized to increase the diffraction efficiency of the incid