Abstract: Through comparing the concentration and inventory of soil organic carbon (SOC) and soil enzyme activity under different plant ages (1, 3, 4, 5 to 8 years) of alfalfa at the foot of Helan mountain in Ningxia, the SOC and soil enzyme activity vertical distribution characters were explored. 1)With the increase of soil depth and plant time, from 0 to 100 cm soil mean SOC concentration were 4.519,4.865,5.120,5.348 and 3.334 g/kg from 1 to 8 years, the soil profile SOC concentration mainly showed a significant accumulation in the 0 to 40 cm depth, accounted for 69.7%, 65.8%, 73.8%, 70.0% and 67.2% from 0 to 100 cm, but it sharply declined below 40 cm;2)0-100 cm soil mean SOC density were 1.148, 1.217, 1.231,1.398 and 0.840 kg/m² from 1 to 8 years, and the surface soil (0-40 cm) accounted for about 54.8% to 61.8%, the highest SOC content and its density was five-year alfalfa, the order of the concentration and density of SOC in the 0-100 cm soil layer were 8 years (19.9 g/kg and 5.04 kg/m²)<1 year (27.7 g/kg and 6.77 kg/m²)<3 years (29.7 g/kg and 7.26 kg/m²)<4 years (30.4 g/kg and 7.38 kg/m²)<5 years (32.2 g/kg and 8.53 kg/m²). 3) Three kinds of soil enzyme activity decreased with the increase of soil depth and plant time, the soil surface (0-10 cm) and subsurface (10-20 cm) enzyme activity decreased significantly, soil enzyme activity was mainly concentrated in the 0-20 cm depth, showed the surface gathering.