Spectrometers with a diffraction grating of the visible and near IR range have a feature that consists in superimposing a spectrum of the second diffraction order on the first. This leads to distortions of the recorded spectrum in its long-wave region. A second-order diffraction correction method is proposed, based on the representation of the spectrum as the sum of Gauss functions, the parameters of which are determined experimentally for individual wavelengths with subsequent numerical interpolation. Parameterization of the line shape in the second order is also used. The details of the required experiment are described and the method is tested on the example of the SSP-600N spectrometer.