The echelle grating spectrometer has cross-dispersion characteristics, and two-dimensional spectral map reduction is the key link to determine its wavelength measurement accuracy, but the changes of spot coordinates caused by environmental changes, processing and mounting have a serious impact on the accuracy of spectral map reduction. In this paper, a spectrum reduction algorithm based on least-squares image coordinate correction is proposed for the middle-step spectrometer. Firstly, the center-of-mass coordinates of the multi-wavelength spot of the calibrated mercury lamp light source are extracted, and the coefficient matrix is constructed by using the theoretical and actual image point coordinates. The translation, scaling, and rotating coefficients of the two-dimensional image plane are obtained by the least-squares estimation method, and then a polynomial fitting is adopted to reduce the influence of residuals, to achieve the correction of the image coordinates of the spot at different wavelengths, and then achieve the accurate resolution of wavelength, and realize the wavelength accurate solving. The experimental results show that the algorithm can effectively improve the spectral image reduction accuracy of the middle-step spectrometer, and the deviation of the corrected coordinates from the ideal coordinates is less than 0.6 image element under the condition of simulating larger mounting errors, which proves that the algorithm has high accuracy.