The uniformity of focal-spot pattern is analyzed by the simulation employing spectral dispersion and distributed phase plate in the laser chain. The uncontrollability of intensity at non-sampling points in the distributed phase plate design causes the loss of uniformity. The simulation confirms the improvement of target irradiation by spectral dispersion; the nonuniformity of focal spot decreases from 58.30% to 19.50%. The relation between nonuniformity and integration time shows that the optimal integration time is 5~6 modulation periods of spectral dispersion. The high spatial frequency of focal spot caused by intensity at non-sampling points is reduced by spectral dispersion smoothing; the intensity fluctuation within 26.3 μm can be eliminated. The results above are confirmed by simulation and experiments, providing theoretical guidance for the next design of distributed phase plate combining with spectral dispersion.