Abstract: This study focused on the changes of cell membrane permeability and activities of protective enzymes at two developmental stages in soybean leaves of two pot-grown soybean varieties under different supplementation of magnesium concentration(Mg~+ 0.00、0.01、0.10、1.00、10.00、100.00 mg/L) using solution culture. The results indicated that membrane permeability (MP), malondialdehyde (MDA) significantly rose under the stress of Mg, while and catalase (CAT) activity declined. Meanwhile, the active oxygen that was produced by Mg stress leaded to an increase of superoxide dismutase (SOD), peroxidase (POD) activities. Application of Mg decreased the MP and the MDA content and raised the CAT activity significantly indicating that Mg could improve anti-oxidant capacity of soybean leaves, among which the effects at Mg+ 1.0010.00 mg/L was the best. Under this condition, the MP and the MDA content of soybean leaves and the activities of SOD and POD were the lowest, while the CAT activity were the highest. Mg deficiency inhibited the CAT activity that could be rectified by rational Mg application. We could conclude that rational application of Mg was favor of clearing up of active oxygen and proving stress tolerance ability. In addition, comparing SOD and CAT at these two stages, both them declined at full bloom stage. This means that the overabundance active oxygen produced in the leave cells of soybean have already beyond the recovery capacity of enzyme resulted in the harm of enzyme, while the POD activity changed slightly indicating POD was of greater stress tolerance capacity and was the dominant protective (enzyme) to eliminate the active oxygen. Therefore, the protective enzymes in the soybean protective system was harmonized each other and synergized against the oxidization.