The allelopathic potentials of aqueous extracts of Wedelia trilobata L. on Brassica parachinensis Bailey and its physiological mechanism of action were studied. The results showed that the aqueous extracts of root, stem, leaf and the whole plant of W. trilobata reduced the seed germination percentage of Brassica parachinensis. Watering Brassica paraohinensis seedlings with the aqueous extracts of W. trilobata resulted in the reduction of fresh root mass, fresh mass of aerial part and the plant height. It also reduced the root activity, leaf chlorophyll content and net photosynthetic rate. The aqueous extracts significantly inhibited the activities of protective enzymes, such as catalase, superoxide dismutase and peroxidases. They also significantly decreased the activities of nitrate reductase,the protective capability of plant itself, and inhibited nitrogen metabolism in plant.