Using large waterproof installations to regulate different soil water contents strictly, we studied effects of spring drought,summer drought and sustained mild drought on the activity of cell defense enzymes in flue-cured tobacco leaves. The results indicated that the superoxide dismutase (SOD) activity increased first and then declined under all kinds of drought stress, the activity of peroxdase (POD) and catalase (CAT) showed a trend of increasing-declining-increasing under spring drought and sustained mild drought, while increasing first and then declining under summer drought. The activity of cell defense enzymes was most affected by summer drought during three treatments.