The influence of beam smoothing on third harmonic generation (THG) in SGII upgrade is analyzed with the nonlinear coupled wave equation. Simulation results show that the continuous phase plate (CPP) designed for 400 μm focus spot causes the conversion efficiency decreasing by 1.4% or so, and the contrast of near field increasing by 0.02 or so in theory, which is within the acceptable tolerance. While the focus spot reaches 800 μm, the conversion efficiency decreases by about 6% and the contrast increases by 0.05 or so, which is out of the acceptable tolerance, and the scheme of the CPP should be improved. The 60 GHz bandwidth of smoothing by spectral dispersion (SSD) causes the conversion efficiency decreasing by about 5%, with obvious temporal intensity modulation, but influence of the divergence caused by SSD is very small. To maintain the stability of the THG, the bandwidth should be reduced reasonably at high fluence operation.