Mask copying process is often used in the LED industry since it can be produced in large quantities with low cost. Influenced by the flatness of the glass substrate, the optical diffraction effect will cause the distortion of the micron-level graphics in the mask copying process. Aiming at eliminating the graphic distortion in the mask copying process, a method was proposed by adjusting the lithography process and resist thickness. The experiment realized the mask copying process for circular graph with the size of 4μm. The experimental result shows that this method can improve the process of mask copying for micron-level graphics and reduce the cost significantly.