[Objective] The purpose of this study was to investigate the mitigation effect and physiological mechanism of different concentrations of salicylic acid (SA) on high temperature stress injury of watermelon seedlings, and to provide a theoretical basis for the rational application of SA in watermelon cultivation and production. [Methods] Potted ‘K53’ watermelon seedlings in greenhouse were used as experimental materials. After spraying different concentrations (0,0.5,1.0,1.5,2.0 mmol/L) of SA on the leaves, the seedlings were stressed at 42°C for 36 h day and night. The growth characteristics, anatomical structure, physiological and biochemical indexes of watermelon seedlings were observed, and the mitigation effects of each treatment were comprehensively evaluated by membership function method. [Results] After high temperature stress, the leaves of watermelon seedlings wilted seriously, the growth of root length was inhibited, the leaves became thinner, and the mesophyll cell structure was damaged. After SA application, the leaves became thicker, photosynthetic pigment content, photosynthetic gas exchange parameters, antioxidant enzyme (POD, SOD, CAT) activity and osmotic adjustment substances (proline, soluble sugar, soluble protein) content increased significantly, MDA content and relative conductivity decreased significantly. Compared with the high temperature stress treatment group, the MDA content of leaves under 1.0 mmol/L SA treatment was significantly reduced by 33.70%, the relative conductivity was significantly reduced by 58.58%, the contents of chlorophyll a and chlorophyll b were significantly increased by 22.71% and 21.63%, respectively. The net photosynthetic rate was significantly increased by 98.34 %, and the activities of SOD and CAT were significantly increased by 23.98% and 19.71%, respectively. [Conclusion] Pre-spraying different concentrations of SA could alleviate the damage of watermelon seedlings under high temperature stress, and 1.0 mmol/L SA treatment had the best effect.