Lithography process is the most important part of mask manufacturing processes. Diffraction mura， as a common defect in lithography process， seriously affects the quality of customer products. Based on the generation mechanism of diffraction mura， the improvement measures are put forward from the aspects of graphic design and chamber environment. Different overlap ratio of overlap and graphics， different overlap position types， chamber temperature and relative humidity fluctuations are set to observe the variation of diffraction mura. The results show that the severity of diffraction nura increases with the increase of overlap ratio； Compared with mixed overlap position type， Single overlap position type performs better on diffraction mura； When the chamber temperature fluctuates more than 0.01 ℃ and the relative humidity fluctuates more than 32%， the fluctuation of focused air flow exceeds the standard， and the diffraction mura becomes more serious. By controlling the temperature and humidity of graphic design and lithography， the occurrence of diffraction mura can be reduced to a certain extent， which can also be used for reference for the improvement of other stripes.