Abstract: After submerging Agropyron mongolicum Keng seedlings roots with PEG-6000 solution, we studied the effect of water stress due to PEG-6000 on activity of protective enzymes (SOD, POD, CAT) in Agropyron. The results showed that, in slight water stress, the Agropyron seedlings could keep the balance of production and scavenging of active oxygen by heightening activities of the enzymes, and reduce the oxidic stress resulting from osmotic stress, and reduce the content of MDA and permeability of cell membrane; but in middle or severe stress, activities of the enzymes decreased, even though some of activities of the enzymes increased, it could not keep the balance of production and scavenging of active oxygen. The content of MDA and permeability of cell membrane strikingly increased and the seedlings were damaged heavily.