The effects of 0.00, 0.05, 0.10, 0.50, 1.00, 4.00 mmol/L dioctyl phthalate (DOP) and diisobutyl phthalate (DIBP) on the growth and physiological and biochemical characteristics of the ‘Zaojia 84-24’ cultivar of watermelon seedlings were studied with substrate culture. The results showed that DOP and DIBP had the strongest promoting effect on the growth of watermelon seedlings when the treatment concentrations were 0.10 mmol/L and 0.05 mmol/L respectively. On the 5th and 10th day after treatment, the activities of SOD, POD and CAT in watermelon seedlings increased with the increase of treatment concentration. On the 15th day after treatment, the activities of SOD, POD and CAT decreased significantly and MDA content increased sharply. In addition, the proline content in watermelon seedlings increased significantly compared with the control on the 15th and 10th day after treatment of DOP and DIBP, respectively. It suggested that the allelopathy of DOP and DIBP was related to the action time and concentration, the higher their concentration, the longer duration of the action, and the stronger allelopathic inhibition effect on the watermelon seedlings.