In this paper, by using the hybrid algorithm merging hill climbing with simulated annealing, the precise design of the circular symmetrical diffractive optical element (DOE) for beam smoothing is completed. By comparing between the designed results of different sampling interval on focal plane, it is found that the facticity of designed result is greatly influenced by the sampling intervals. During the designing process, it is insufficient to sample only according to the sampling theory. To get the faithful intensity distribution for beam smoothing, the sampling interval must be less than or equal to 1/2 of the traditional sampling interval. Only under this condition, both the intensity of samples and other points on focal plane can be controlled to meet the requirements of beam smoothing.