For improving the uniformity of the focal spot in the inertial confinement fusion (ICF) experiments, we simulated the effects of the different spectral shapes of broadband light with the focused spot through phase plate, such as random phase plate (RPP) or continuous phase plate (CPP), and the uniform bandwidth and maximal dispersive angle. The power spectral density (PSD) and root mean square (RMS) of the focused spot intensity were used to evaluate smoothing effect. The results show that the optimized spectrum can improve smoothing efficiently, and the RMS can be improved by 43.7% compared with the sinusoidal phase modulated spectrum. Generally, the smoothing efficiency would be better if the weighting of the different spatial frequencies of the focused spot was considered to design phase plate.