The focal plane intensity distribution, realized by the diffractive optical element (DOE) and the smoothing by spectral dispersion (SSD) technique, is theoretically analyzed based on the spatial frequency spectra. The focal plane intensity distributions of the DOE can be expressed as the sum of a series of sine and cosine functions with different harmonic spatial-frequencies and complex amplitudes, no matter whether the SSD technique is adopted or not. Theoretical analysis shows that the spatial frequency spectra are modulated by the SSD technique, and the modulation is determined not only by parameters of the SSD technique, but also by phase distribution of the DOE. The SSD technique smoothes the focal plane intensity distribution of the DOE. But to obtain good beam smoothing performance, the DOE should be optimized considering the parameters of the SSD technique. Finally the DOE is optimized at a certain choice of SSD parameters. The simulated results show that the tolerance of the DOE to incident wave-front distortion is improved with the use of the SSD technique.