The relationship between grating parameters and wavelet spectrum values was deduced. Firstly, the vector diffraction theory and scalar quantity diffraction theory were used to calculate the diffraction efficiency. The approximately equal conditions were analyzed next. Finally, the relationship was obtained. The gap width of the plane transmission grating was calculated with the Mexicohat wavelet. The relationship among the parameters of the plane transmission grating, diffraction efficiency and Mexicohat wavelet values was obtained.