Effects of adding lignite on the spectral properties of dissolved organic matter (DOM) in sheep manure organic fertilizer were obtained, which provided a basis for the evaluation of the maturity of high humic acid organic fertilizer. Inner Mongolia lignite and sheep manure were used as raw materials, 10% lignite was added to ferment organic fertilizer of sheep manure, and the DOM extracted from organic fertilizer samples at different stages was analyzed by UV-visible spectrum analysis, fluorescence spectrum analysis and Fourier Transform infrared spectroscopy. Ultraviolet-visible absorption spectroscopy analysis showed that the E465/E665 value of organic fertilizer samples decreased first and then increased with the fermentation of organic fertilizer. From 9.110 4 at the beginning of organic fertilizer to a minimum of 4.647 7, and then to 5.390 1 at the end of organic fertilizer, A1 showed a decrease and then increased, and both A2 and A3 showed a trend of increasing first and then decreasing, and the peak appeared at 12 d. Synchronous fluorescence spectroscopy showed that the ratio of fluorescence peak intensity (I470/I435) increased from 0.452 8 starting from organic fertilizer to 0.655 2 at the end of organic fertilizer, and AHLR/AFLR showed an upward trend, rising from 0.673 9 for organic fertilizer to 1.040 8 for organic fertilizer. After 18 d of organic fertilizer fermentation, the fluorescence intensity of the organic fertilizer after fermentation was lower than that of the organic fertilizer at the beginning, and the relative fluorescence intensity of the 10% lignite-added sheep manure organic fertilizer was significantly higher than that of the un-added lignite. Fourier transform infrared spectroscopy showed that carbohydrates and proteins in DOM were gradually decomposed during the fermentation of organic fertilizer for 18 d, while the content of carboxyl groups and benzene ring-containing substances increased significantly. With the progress of fermentation, the non-humus substances in DOM are converted into humus-like substances, the degree of polymerization or aggregation of unsaturated structures becomes larger, and the stability increases. The addition of lignite can effectively promote the decomposition of lignin substances, improve the degree of aromatization of DOM and accelerate the maturity of organic fertilizer.