Abstract: Abstract: In this paper, the root growth and protective enzymes of American ginseng after continuous cropping were studied to reveal the mechanism for the effects of continuous cropping on the American ginseng physiology. The root weight, root vigor and root enzyme activity containing the ascorbic peroxidase (APX), the catalase (CAT), the peroxidase (POD), superoxide dismutase (SOD), glutathione reductase (GR), and malondialdehyde (MDA) were carried out after three months American ginseng was planted in new soil, one- year, two- year and three-year continuous cropping land, respectively. American ginseng’s root weight in two- year, three- year, four-year planting land was 0.76, 0.83, 0.68 times of that in new soil respectively with significant differences (P<0.05). Root vigor was 1.30、1.35、1.65 times compared with the control (P<0.05). The differences of APX activity (3.52, 0.25, 0.12 times) was significant. SOD activity (0.72, 0.36, 0.37 times). CAT activity (1.86, 0.79, 0.74 times) in two-year continuous crop was remarkable (P<0.05), while three-, four-year continuous crop didn’t vary significantly. POD activity of two-year, three-year continuous crop (2.22, 2.50 times) showed significant differences while no significant distinction was detected in four-year continuous crop (1.43 times). which was similar variation to GR activity (2.57, 2.75, 2.41 times), Significant distinction of MDA was revealed in four-year continuous crop (1.18 times), but no distinction in two- year, three-year continuous crop (1.05, 1.06 times) was gained. With the increasing continuous cropping year, root weight declined, and root vigor went up. APX, CAT, POD and GR activity firstly rose and then decreased. SOD activity showed downtrend. MDA content was remarkably rising in four-year continuous crop.