In the process of surface acoustic wave(SAW) device photolithography, the line width of the photoresist is inconsistent with the photomask, especially, the line width variation of the non-uniform lines with different widths has a deviation after photolithography. The influence of proximity exposure diffraction effect on the line width is studied in this paper. The relationships among the parameters such as line and gap widths on the photomask, diffracted light intensity, spacing between photomask and photoresist, etc are analyzed. The results show that the precise control of the non-uniform line width of SAW devices can be realized by using the method of calculating the variation value of different line widths after exposure by establishing an optical model, and by using the programming method to compensate the lines with different sizes and the gap widths in the photomask data file.