In this paper, the performance of the diffractive optical element (DOE) for beam smoothing is analyzed by using the spatial frequency spectrum. Two parameters, light efficiency and top non uniformity, are re defined based on the spatial frequency spectrum. These two parameters can truly and exactly evaluate the performance of the DOE for beam smoothing, because their definitions go back to the origin. Finally, these two performance parameters are calculated, whether the precise design of the DOE for beam smoothing is adopted or not. The calculated results show the validity of the precision design of the DOE.