Abstract: ‘Aoyue lettuce’ was used as the experimental material.The potted method was adopted, four treatments were set up including the nutrient solution（CK）,the nutrient solution containing 100 μmol·L^-1 melatonin（MT）,the nutrient solution containing 25% PEG（D） and 25%PEG nutrient solution + root irrigation nutrient solution contained 100 μmol·L^-1 melatonin（D+MT）, and the effects of melatonin on the growth and physiological characteristics of lettuce seedlings under PEG stress were studied, in order to provide reference for drought-resistant cultivation of melatonin in lettuce.The results showed that root length, root volume, root area, average root diameter, aboveground and underground dry weight and root-shoot ratio of melatonin applied lettuce seedlings were significantly increased in melatonin applied to roots under drought stress（P＜0.05）,leaf relative water content also increased, but the difference was not significant.Melatonin increased the actual photochemical efficiency and photochemical quenching coefficient of the photosystem Ⅱ of lettuce leaves, and decreased the non-photochemical quenching coefficient of PS components in leavesunder drought stress.In addition to root peroxidase（POD） activity, superoxide dismutase（SOD） activity, peroxidase（POD） activity and soluble protein content in leaves and roots were increased by melatonin under drought stress, but malondialdehyde（MDA） contentin leaves and roots decreased.SOD activity in roots and leaves was significantly different between D and D+MT（P＜0.05）.Therefore, root application of 100 μmol·L^-1 melatonin can regulate the morphological characteristics of lettuce seedlings, enhance the activities of protective enzymes and soluble protein content in roots and leaves, and improve photosynthetic index of lettuce leaves, alleviating the stress of lettuce under drought conditions, thus providing theoretical basis for high yield cultivation of lettuce.