Machine-ruling method is one of the important methods for making plane grating. In order to improve the grating′s performance and application level, it is particularly important to analysis the influence of ruling line′s equal-distance curve and location error on plane grating performance, such as resolution, scattered light, etc. According to Fermat principle, a light-tracing mathematical model that simulates collimated monochromatic light incident on a plane grating which contains line′s curved error, and location error and corresponding diffraction light imaged on the focal plane is established. The influence of above-mentioned grating line errors on grating performance is studied. Results show that grating line′s curve and location error mainly influence grating sagittal and meridional spectral performance, separately, line′s curved error has little effect on grating resolving power and scattered light. According to the analysis above, the ruling system of grating ruling machine is improved. Statistical average value of grating line′s location error caused by modified ruling system is reduced to less than half of the original amplitude. Thereby grating scattered light is effectively suppressed.