Abstract: In order to provide scientific theoretical basis for the application of Tagetes patula in the garden plants configuration, the aqueous extracts from Tagetes patula was used to search the allelopathic effects on seed germination and seedling growth of Dianthus chinensis, Achillea millefolium, Orychophragmus violaceus, Celosia cristata using petridishs with filter papers. Results showed that the aqueous extracts from Tagetes patula had significant allelopathic effects on seed germination and seedling growth of four garden plants. The aqueous extracts from Tagetes patula inhibited seed germination of D. chinensis, O. violaceus and C. cristata, on the contrary, inceased seed germination of A.millefolium in a degree. The extracts also inhibited seedling growth of D. chinensis at all concentrations, but promoted at the lower concentrations and inhibited at the higher ones the seedling growth of O. violaceus and C. cristata.The inhibition was the most intensive by treatment with 100.0 mg/mL of the aqueous extracts. Whereas, the growth of A.millefolium was promoted significantly at 12.5, 25.0, 50.0 mg/mL, and inhibited only by treatment with 100.0 mg/mL. The chlorophyll content in these four garden plants were reduced by 23.46%, 42.56%, 14.75% and 58.28%, when the aqueous extracts concentration was 100.0 mg/mL. The extracts increased the activities of peroxidase (POD) of O. violaceus and C.cristata at the lower concentration, while decreased the activities of POD at 100.0 mg/mL. The activity of POD of D. chinensis declined at all concentrations, while A.millefolium declined earlier then ascended later. The content of MDA of D. chinensis and C.cristata ascended at all concentrations, whereas A. millefolium and O. violaceus declined earlier then ascended later. It implied that T. patula had different allelopathic effects on different garden plants and “concentration effect” was found in this research.