Forest soil contains lots of carbon, which presented in different fractions with various functions. This study collected surface and subsurface soil samples from brown soil and cinnamon soil under 45 years old Robinia pseudoacacia forest. Difference within soil humic substances under long-term forest, the influence from soil type and depth were studied in this paper. Surface and subsurface soil samples collected from cinnamon and brown soil were defined as CO and CA, BO and BA, respectively. Humic substances were separated as fulvic acid (FA), humic acid (HA) and humin (HM) and analyzed by Fourier transform infrared spectroscopy (FT-IR). Results showed that FA contained mainly stretching vibration of carbohydrate H-bonded OH at 3 400 cm-1, aromatic CC at 1 655 cm-1 and C—OH stretching of aliphatic at 1 110 cm-1. Compared with FA, HA has weaker peaks at similar wave numbers than those in FA. HM fraction has stronger peaks at 1 110 and 1 030 cm-1 than HA, which assigned as C—OH stretching of aliphatic OH and C—O stretching of polysaccharide or polysaccharide-like compounds. Results also showed that more peaks were collected in surface soils and the absorption intensity are also stronger than those in subsurface soils. In addition, absorption bands richness and intensity of FA fraction suffered less influence from soil type and depth than those in HA and HM fractions. Through analysis of absorption bands, FA fractions had strong aromatization, while aromatization of HA and HM were only stronger under surface soil. The aromatic and aliphatic structures in HA fractions under BO soil were much stronger than those in cinnamon soil. Thus, we conclude that humic substances in 45 years old Robinia pseudoacacia forest soils is influenced by soil depth and soil type, especially in HA and HM fractions.