Soil samples taken from a long term fixed spot experiment (21 years) with different fertilization were used to study the
content and oxidation stability of soil organic matter, the energy state, structure and fractions of soil humic acids. The main results showed that compared with the content of organic matter before experiment, the soil organic matter content decreased by 1.04 g/kg with no fertilization ,and decreased by 0.48 g/kg with fallow, application of chemical fertilizer for 21 years could maintain the level of soil organic matter, and applying organic materials (yard manure or corn straw) caused organic matter to be increased by 1.17～2.85 g/kg. Application of organic materials reduced the oxidation stability of soil organic matter as compared with no fertilization. The opposite was true to the treatments of chemical fertilizer or fallow. After 21 years, the soil humic acid energy state was lower than that after 10 years. The energy state order of different treatments was yard manure>straw>no fertilization>chemical fertilizer>fallow. Application organic materials for a long time made the molecular of humic acid become simpler, and increased the humic acid activity and its simple fractions. The results of application of chemical fertilizer was on the contrary.