Abstract: Spectra measured using an atmospheric carbon dioxide grating spectrometer (ACGS) by the Chinese global carbon dioxide monitoring scientific experimental satellite (TanSat) in the bands of 0.76, 1.61, and 2.06 μm can be used for retrieving carbon dioxide (CO₂) concentrations by fitting observations and simulations using an optimal estimation algorithm. Accurately detecting the change in the center wavelength is important because of its very high spectral resolution and accuracy requirement for product retrieval. Variations in the center wavelength for all the three bands of ACGS have been calculated at the locations of the Fraunhofer lines by comparing solar-viewing measurements and a high-resolution solar reference spectrum. Variations in magnitudes less than 10% of the spectral resolution for each band have been detected. Changes are probably caused by the vibration and instrument status difference between the ground and space, especially the temperature variation in the orbit. The scheme described herein can be used not only for monitoring spectral stability but also to gain spectral knowledge prior to the level-2 product processing. These minor temporal changes in the wavelength in the orbit should be corrected during the product retrieval.