Abstract: The watermelon variety ‘B4’ was selected as material to investigate the effect of salicylic acid（SA） and brassinolide（BR） on seedling physiological characteristics of watermelon under low temperature stress.Six different concentrations of SA(0.01、0.1、0.5、1.0、2.5、5.0 mmol·L^-1) and four different concentrations of BR(0.005、0.05、0.5、5.0 mg·L^-1) were set up to spray on the leaf surface, respectively.In order to elucidate the physiological mechanism of exogenous hormones regulating the growth and development and improving cold tolerance of watermelon seedlings under cold stress, and provide a technical referance for the rational use of SA and BR in the cultivation of watermelon under low temperature stress.The results indicated that the relative malondial dehyde（MDA） content of watermelon leaf decreased firstly and then increased, while the superoxide dismutase（SOD） activity, peroxidase（POD） activity and soluble protein content increased firsty and then decreased with the increase of SA and BR spraying concentrations under low temperature stress.The optimal concentrations of SA and BR to spray watermelon seedlings were 0.5 mmol·L^-1and 0.05 mg·L^-1,respectively.In addition, the SOD activity, POD activity and soluble protein content could be increased significantly, while the MDA content decreased significantly after a period of low temperature treatment.The SOD activity, POD activity and soluble protein content showed a trend of gradual decrease, and the MDA content increased gradually with the extension of low temperature treatment time.The treatment effect of SA was the best on day 0.5 of low temperature stress, while that of BR was the best on day 1.5.