In order to reveal the competitive relationship between invasive weeds and herbaceous flowers, the invasive plant Conyza canadensis was used as the donor material, and the commonly used flowers Cosmos bipinnatus and Brassica juncea L. were used as the receptor materials. The allelopathic effects of the extracts of the roots and leaves of C. canadensis on the seed germination, seedling growth, antioxidant protective enzyme activities (SOD, POD and CAT) and malondialdehyde (MDA) contents of the receptor plants were studied. The results showed that the extracts of C. canadensis had significant allelopathic effects on the seed germination and seedling growth of the receptor plants, which basically showed the trend of " low promotion and high inhibition ". The seed germination rate of C. bipinnata and B. juncea was the highest when the concentration of root and leaf of C. canadensis was 25 g·L-1, and the lowest when the concentration of extract was 100 g·L-1. The extract of C. canadensis has a concentration effect of low promotion and high inhibition on the growth of upper and lower hypocotyls of seedlings, and the greater the concentration, the more significant the inhibition. The activities of superoxide dismutase (SOD) and peroxidase (POD) in rapeseed were significantly higher than those in the control, while the activities of SOD and POD in C. bipinnatus were significantly lower than those in the control. The catalase (CAT) of C. bipinnata and B. juncea showed a trend of slowly increasing first and then decreasing. The MDA content of C. bipinnata seedlings increased gradually with the increase of the concentration of root and leaf extract, and most of them were higher than the control level. The MDA content of B. juncea seedlings increased gradually with the increase of the concentration of root extract, and decreased with the increase of the concentration of leaf extract, but most of them were significantly lower than the control level. The study found that the extract of C. canadensis showed the allelopathic effect of " low promotion and high inhibition " on the seed germination and seedling growth of the two kinds of flowers. The comprehensive effect was that the B. juncea was greater than the C. bipinnata, and the root extract was greater than the leaf extract.