In order to improve the positioning accuracy of the reference position in the grating diffraction laser warning system, a spot center approximation method based on the minimum grayscale difference of indexed ring was proposed. According to the given initial coordinates of the spot center, a 3×3 center coordinate matrix was generated. The Lagrange interpolation was used to calculate the grayscale values of points on the indexed ring centered at the center coordinate matrix. The center coordinate with the minimum grayscale difference on the indexed ring, which represented the true coordinates of the spot center, was approximated using a binary search method. In the accuracy comparative experiment, the proposed algorithm showed average errors of 0.001 3 pixels in the x-direction and 0.001 1 pixels in the y-direction across three sets of ideal spot model samples. In the stability comparative experiment, the proposed algorithm exhibited average standard deviations of 0.165 3 pixels in the x-direction and 0.186 0 pixels in the y-direction across three sets of actual spot image samples. The spot center approximation method based on the minimum grayscale difference of indexed ring has obtained the lowest average error or standard deviation in two comparative experiments, respectively, which demonstrates higher accuracy and better stability, thereby improving the positioning accuracy of the reference position in the laser warning system, and provides a new solution for spot center positioning algorithms.