In order to understand the interaction of Thalia dealbata with other aquatic plants, we studied the effects of the aquatic extract from rhizome and root of T. dealbata on the growth, net photosynthetic rate, root activity, chlorophyll content, and antioxidant defense system in seedlings of Nymphoides peltatum, Vallisneria natans, Nasturtium officinale, Phragmites australis, Iris pseudacorus by using sand culture method, and analyzed the chemical components of the extract using GC-MS. Results showed that the aquatic rhizome and root extract of T. dealbata had significant allelopathic effects on seedling growth in N. peltatum, V. natans, N. officinale, Ph. australis and I. pseudacorus, with the effects varying with the type of pasture plants and the concentration of the extract. The seedling growth index in N. peltatum, V. natans, N. officinale, Ph. australis and I. pseudacorus was highly significantly inhibited by treatment with 50mgDw/mL of the aquatic extract; the net photosynthetic rate in the seedlings of V. natans, N. officinale and I. pseudacorus were reduced by 69.0%, 63.7% and 73.5%; the root activity in the seedlings of N. peltatum, V. natans, N. officinale, and I. pseudacorus were reduced by 67.3%, 65.4%, 52.2% and 46.7%, respectively, and the chlorophyll content in the five aquatic plants were reduced by 59.7%, 71.2%, 35.2%, 50.0% and 76.5%, respectively, compared with the control. At 5mgDw/mL, the extract significantly increased the activities of superoxide dismutase(SOD), peroxidase(POD), and catalase(CAT); whereas at 50mg Dw/mL the extract reduced the activities of those enzymes while significantly increased the content of malondialdehyde(MDA)in the seedling of the aquatic plants. 44 secondary metabolic compounds were identified in the aquatic rhizome and root extract of T. dealbata. The main compounds were guaiacol (78.93%)、dibutyl phthalate (7.13%)、diethoxyethyl phthalate (1.48%), coumaran(1.09%), diethyl phthalate (0.98%), Indole(0.65%), butylated hydroxytoluene(0.64%) and Terpineol (0.70%), which all together accounted for 91%of the total.