Abstract: The paper aims to study the effects of boron stress on active oxygen metabolism and antioxidant system of different genotypes of watermelon cultivars. Seed- used watermelon, watermelon cultivars ‘Heimeiren’and wild watermelon were used to test the activities of SOD, POD, CAT and the content of O2·-, H2O2, MDA under 0.00, 0.05, 0.20, 0.60 mmol/L boron concentration. The results showed that lots of ROC (O2 ·- and H2O2) were produced in watermelon under boron stress, ROC led to MDA accumulation. The high boron stress led to more ROC accumulation in the leaf of three watermelon cultivars. Under boron stress, POD activities were increased in watermelon, CAT activities were decreased. The SOD activities of seed- used watermelon and‘Heimeiren’were reduced. The SOD activities of wild watermelon were increased. Boron stress led to ROC accumulation. ROC accumulation activated the antioxidant system of watermelon. But the antioxidant system of watermelon could not completely resist active oxygen damage under boron stress. Stress resistance of seed-used watermelon was the strongest.