Abstract: The ideal plane-wave or gauss is often used as light source when studying laser system with numerical simulation. The actual light source for laser transmission system does not meet the condition of ideal light, even with poor beam quality, which results in the deviation in the simulation compared with experiments. Starting from the relationship between wave aberration and beam quality , the numeric simulation method of nonideal beam was discussed here. The initial aberration was equivalent phase screen, which intensity was related with beam quality of the light simulated. According to the setting values of , the optical field of light was simulated in the method, which was applied in plane-wave and gauss. Compared with setting values, the beam quality of optical field calculated shows that the method can simulate the light with certain beam quality.