In order to understand the interaction of Artemisia frigida with other pasture plants, we studied the effects of the aquatic extract from stem and leaves of A. frigida on the growth, root activity, chlorophyll content, and antioxidant defense system in seedlings of Melilotus officinalis Lam., Elymus dahuricus Turcz., Agropyron cristatum L. and Leymus chinensis by using sand culture method, and analyzed the chemical components of the extract using GC\|MS. Results showed that the aquatic stem and leaf extract of A. frigida had significant allelopathic effects on seedling growth in M. officinalis, E. dahuricus, A. cristatum and L. chinensis, with the effects varying with the type of pasture plants and the concentration of the extract. The seedling growth index in M. officinalis, E. dahuricus, A. cristatum and L. chinensis was highly significantly inhibited by treatment with 25mgDw?mL-1of the aquatic extract; the root activity in the seedlings of the four pasture plants were reduced by 46.7%, 65.9%, 59.3%, and 66.7%, respectively, and the chlorophyll content in M. officinalis, E. dahuricus, and L. chinensis were reduced by 47.3%, 56.6%, 46.7%, respectively, compared with the control. At 10mgDw?mL-1, the extract significantly increased the activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT); whereas at 25mgDw?mL-1 the extract reduced the activities of those enzymes while significantly increased the content of malondialdehyde (MDA) in the seedling of the four pasture plants. 30 secondary metabolic compounds were identified in the aquatic stem and leaf extract of A. frigida. The main compounds were camphor (27.6%), borneol (10.1%), achillin (9.8%), cineole (8.7%), ledene alcohol (4.1%) and p\|menth-1\|en-4\|ol (41%), which all together accounted for 60% of the total.