The theoretical conversion efficiency of the second harmonic generation of the flattened Gaussian beam's and Gaussian beams was obtained by using the numerical simulation on the crystal CsLiB6O10 in the phase matching type I and type II for the first time. The conversion efficiency versus the crystal lengths, polarization ratio and intensity of the pumping light are given out in this paper, it is pointed out that the SHG conversion efficiency 78.5% of the flattened Gaussian beam is higher than that of the Gaussian beam 69.1% in the type II and the other condition.