Abstract: The paper aims to study soil organic matter of different soil depths under different ages of Caragana microphylla Lam. forest, and provide a theoretical basis for further research on soil organic carbon and regional vegetation construction in transition zone between cropping area and nomadic area in north China. The authors combined the methods of field investigation and laboratory analysis, analyzed soil organic matter of different soil layers under different ages (5, 11, 17, 23 and 29 a) of Caragana microphylla Lam. forest in Aohan Banner, Chifeng sandy soil area. The results showed that soil organic matter of the same age forest under different site conditions first increased and then decreased with the increase of soil depth; soil organic matter under the same site condition of different forest ages followed the order of dune-lowland＞ shady-slope＞ sunny-slope; soil organic matter under the same site condition of different forest ages increased with the forest age.