The diffraction characteristics of 1200 lp/mm ultraviolet ruled grating master and the second duplicate were simulated by differential theory and distinguished. The simulation results mere compared with the tested results. The numerical simulation indicated that the diffraction characteristics of the ultraviolet ruled grating master and the second duplicate are slightly different for the change of grating groove profiles. The methods of grating groove profile polynomial fitting and Fourier series fitting are presented. This theoretical analysis clarified the dispute in the diffraction grating duplication craft, perfected reprint technology and offered practical theoretical reference for improving diffraction grating efficiency.