The principle of non-contact photolithography for reducing the diffraction effect is analyzed systematically in terms of the diffraction theory,and solutions to constructional parameters of optical system for reducing the diffraction effect have been obtained. The relationship among different widths of lines as well as clearances is presented, and it has been proved by a lot of experiments. The new type of optical system has been used in a circular dividing machine, which is useful in obtaining high-quality gratings.