A model of drought was created on pigweed and the effects of drought stress on the activity of acid phosphatase and its protective enzymes were examined. The pot-cultured pigweeds were divided into 4 groups (ten plants per group) when they reached 6 leaves. (1) In the control group, the culture media contained 70 - 85% of field moisture capacity, (2) In the second group, the mild drought stress group, the culture media contained 50 - 60% of field moisture capacity, (3) The moderate drought stress group had a culture media that contained 40 - 50% of field moisture capacity; (4) The severe drought stress group culture media contained 30 - 40% of field moisture capacity. All through the process of the present study, the pigweed plants were cultured under natural conditions on the rooftop of the laboratory building; though transferred indoor in rainy days to avoid the influence of natural precipitation. The plants were sampled and detected every five days after the administration of drought stress. The results clearly demonstrated that the drought stress significantly enhanced the activity of acid phosphatase, membrane permeability and MDA contents; though the activity of acid phosphatase declined after a certain time of drought stress, the extent of membrane permeability and MDA contents still increased with the time. The membrane permeability and MDA contents were correlated with a correlation coefficient of 0.963, 0.971 and 0.939 under mild, moderate and severe drought stress, respectively. The activity of superoxide dismutase (SOD), peroxide dismutase (POD) and hydrogen peroxidase (CAT) was also enhanced with increase in the intensity of drought stress or the prolongation of drought stress at first and then decreased some time afterwards. It was concluded that drought stress enhanced the activity of acid phosphatase, membrane permeability and MDA in the pigweed plants, which was able to resist a certain drought stress by enhancing the activity of protective enzymes. However, excessive drought stress markedly affected the metabolic systems of enzyme and decreased the activity of enzyme.