The study was conducted to determine the inhibition effect of Euphorbia helioscopia L. aqueous extract on Lemna minor L. through measuring by plant growth and antioxidant enzyme system. A total of 18 treatments were successively divided into 3 experimental groups according to the content of tea saponin added as extraction adjuvants (group 1: no tea saponin,group 2: 0.2g/L tea saponin,group 3: 0.5 g/L tea saponin). Each group was set up on the six concentration series of E. helioscopia aqueous extract as 0.001, 0.003, 0.005, 0.01, 0.015, 0.02 g/mL. A blank control was also set up. The results showed that the injury degree of L. minor was directly correlated to the concentration of E. helioscopia aqueous extract. The antioxidant enzyme system was affected at low E. helioscopia aqueous extract concentrations (0.001, 0.003, 0.005 g/mL). The CAT, POD activity decreased, and SOD activity was reduced after an initial increase while apparent injury symptoms were not observed. L. minor was seriously injured apparently at higher concentration (0.01, 0.015, 0.02 g/mL) and 90%-100% individuals were injured after 36 hours stress. Both the quantity and the injury degree of injured individuals were positively correlated with the concentration of E. helioscopia aqueous extract. L. minor in groups of E. helioscopia tea saponin extract were injured mildly compared with those in the group with only aqueous extracts. A component analysis of E. helioscopia extract is necessary to find the differences between group 1, group 2 and group 3, and it will help to identify the active ingredient inhibiting L. minor.