Grazing has a lot of effects on grassland soil properties. The objective of the present study is to investigate the effects of different managements on soil humic acids. Soil samples were collected from the surface soil of four adjacent plots (Ⅰ, Ⅱ, Ⅲ and Ⅳ) at a long-term experiment site of the Inner Mongolia Grassland Ecosystem Research Station (IMGERS), the Chinese Academy of Sciences. The structure features of humic acids were investigated by cross-polarization magic angle spinning 13C-nuclear magnetic resonance spectroscopy (CP/MAS13C-NMR), FTIR and elemental analysis. The results indicated that the humic acid extracted from the grazing plots were characterized by a higher degree of humification, including loss of polysaccharides, decrease in lignin content and increase in aromaticity. Compared with the humic substances from the grazing plots, the humic substances from fenced plots showed a lower degree of humification and were considerably more aliphatic in nature.