The importance and complexity of soil organic matter have been a hot topic both at home and abroad. As the main body of soil organic matter, soil humus is an important material foundation of soil fertility and plays an important role in soil nutrient cycling and soil structure. Humin is an important component of soil humus, which accounts for the majority of soil organic C and organic N. It is also a relatively stable humus component. Humin plays an important role in the fixation and availability of nutrient elements (C, N, S, etc.), soil fertility and ecological environment. Based on the long-term localization test of black soil (which began in 1979), the variation characteristics of Hu molecular structure were analyzed by differential thermal analysis, infrared spectroscopy and nuclear magnetic resonance spectroscopy. The results showed that all fertilization treatments could significantly increase the content of soil organic carbon, and organic and inorganic fertilizers had the most significant effect. There were differences in Hu content among different fertilization treatments, but not significant. The thermal properties of soil Hu showed that the decomposable organic matter and aliphatic structure were higher when organic manure was applied, while the decomposable organic matter and aromatic structure were higher when chemical fertilizer was applied alone. The infrared spectra of Hu also showed that the ratio of Hu in soil increased by 2 920/1 620 with the application of organic fertilizer and organic-inorganic fertilizer alone, and the aromaticity of Hu was weakened. The effect of applying organic fertilizer alone on increasing the ratio of aliphatic chain hydrocarbons in Hu in soil was higher than that of other treatments.13C NMR spectroscopy analysis showed that compared with CK, organic and inorganic fertilizers could improve the stability of soil organic carbon, while the decomposition degree of organic matter increased and the stability decreased under the single chemical fertilizer treatment.