Abstract: The reason is theoretically discussed for the existence of sub-diffractive spots around the focal spot and the decrease of diffraction efficiency in beam smoothing by 320 mm-diameter diffractive optical elements. Through the simulation analysis, it is suggested that, by means of adjusting the design parameters, the intensity of sub-diffractive spots can be suppressed effectively, which can ensure the effect of diffractive optical elements in beam smoothing. For example, by appropriately increasing sampling points, the proportion of focal spot to whole output range, named Yita, can be decreased, and the efficiency loss engendered by sub-diffractive spots would be restricted. When Yita is less than 0.2, the efficiency loss can be controlled within 2%. The experimental results with 70 mm- and