From the error analysis of multi-level diffractive optical elements (MDOE) in etching process, the parameter of error skewness was suggested to reflect the overall etching error. The variation of error skewness versus the beam shaping effect of diffractive optical elements is investigated, and a flat interval which can reduce the impact of etching error existed in the curve of error skewness. According to these curves, a technical method for improving the factual illumination quality of MDOE was proposed by controlling etching depth. The experimental results show that the peak value (PV) of the optical field parameter reduces about 30% after the technical optimization.