Effects of potassium on protective enzyme activities, active oxygen metabolism and lipid peroxidation in roots and leaves of of Panax ginsneg C. A. Meyer were studied by hydroponics method. Four potassium levels of 0.0 mmol/L, 3.0 mmol/L, 6.0 mmol/L, 12.0 mmol/L were investigated. Results showed that the responses of protective enzyme system under the different potassium levels were different. The SOD activities of leaves in the P. ginsneg showed an increasing tendency. The activities of SOD（root）, POD（roots and leaves） in the P. ginsneg increased at first, and then decreased with the increase of potassium levels, the highest activities reached at 3 mmol/L, and then presented as passive tolerance. The activities of SOD, CAT and POD in the leaves were significantly greater than those in the roots. The CAT activities were entirely different from SOD and POD, it decreased with the increase potassium levels until 6.0 mmol/L, and then increased. CAT played the mainly role under the deficient potassium levels and high potassium levels. MDA produced by membrane lipid peroxidation decreased with the increase of potassium concentration until 6.0 mmol/L, and then increased. Protective SOD, POD and CAT enzymes coordinated with each other to resist reactive oxygen species and promote healthy growth.