To improve the ability of beam smoothing in the device of inertial confinement fusion (ICF) in China, the theory of smoothing by spectral dispersion (SSD) is studied, and some experiments are carried out to analyze the mechanism of SSD in contrast with another technology of smoothing—distributed phase plane (DPP). It is found that the uniformity of focal spot in direction of dispersion is improved markedly using sinusoidal phase modulation (PM) and dispersion in SSD. And the intensity modulation of focal spot in this direction is smoothed by SSD. According to the experiments results, the changes of spectral intensity of spatial frequency at low-mode are very limited, and at high-mode are obviously; the nonuniformity varies from 40% to 12.5%. The ability of smoothing depends on the parameter of SSD. It also shows that the color cycle is very important in the process of smoothing, although the smoothing by broaden of laser spectral width for PM is poor.