This study investigated the effects of exogenous 24-epibrassinolide (EBR) on the leaf morphology and photosynthetic characteristics of tomato seedlings (cv. Jiersi) cultured hydroponically under low light stress. The results showed that low light stress induced adaptive changes in the leaf morphology of tomato seedling. Low light stress significantly increased the leaf area, specific leaf area, angle between stem and leaf, vertical angle and cable bent, but significantly decreased the dry mass of leaf. Low light stress also significantly decreased the maximum net photosynthetic rate,. apparent quantum yield, dark respiration rate, carboxylation efficiency and Rubisco large subunit content. The light compensation point and carbon dioxide compensation point were dramatically enhanced under low light stress. However, foliar spraying EBR significantly increased the leaf area, leaf dry mass, angle between stem and leaf, vertical angle by 14.1%, 57.1%, 12.3% and 7.7% under low light stress, respectively. EBR significantly decreased the specific leaf area and cable bent by 30.5% and 10.6% in low light stressed plants. In addition, EBR significantly enhanced the apparent quantum yield, dark respiration rate and carboxylation efficiency by 20.4%, 17.9% and 9.3%, respectively, but significantly reduced the light compensation point and carbon dioxide compensation point by 21.9% and 4.3% under low light stress. Moreover, EBR also significantly increased the Rubisco large subunit content in low light stressed leaves. These results suggested that application of exogenous EBR could effectively alleviate the inhibition of photosynthesis induced by low light stress via improving the apparent quantum yield, dark respiration rate, carboxylation efficiency and Rubisco content, and maintaining the stability of leaf morphology, thus low light tolerance of tomato seedlings.