Diffraction optical waveguides have the advantages of compact size and light weight for augmented reality displays. The period of subwavelength gratings on the surface of grating waveguides significantly affects imaging quality, making it essential to achieve fast and accurate measurements of grating periods. The Littrow diffraction method has the advantages of low cost and non-destructive properties. However, it requires precise alignment of the grating sample, necessitating multiple adjustments prior to measurement, which hampers rapid evaluation. To address this problem, this study analyzes the diffraction characteristics of misaligned gratings and proposes an image processing-based method to calculate the misalignment angle and correct measurement errors. The proposed method enables accurate measurement of the grating period under conditions in which the grating pose is not strictly aligned, thereby enhancing measurement efficiency, it is expected to achieve facilitating rapid detection of augmented reality grating waveguides.