Abstract: 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 is proposed. According to the given initial coordinates of the spot center, a 3x3 center coordinate matrix is generated. Lagrange interpolation is 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 represents the true coordinates of the spot center, is approximated using a binary search method. In the accuracy comparison experiment, the proposed algorithm in this paper showed average errors of 0.0013 pixels in the x-direction and 0.0011 pixels in the y-direction across three sets of ideal spot model samples. In the stability comparison experiment, the proposed algorithm in this paper exhibited average standard deviations of 0.1653 pixels in the x-direction and 0.1860 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 obtained the lowest average error or standard deviation in both comparative experiments. It demonstrates higher accuracy and better stability, thus improving the positioning accuracy of the reference position in the laser warning system. This method provides a new solution for spot center positioning algorithms.