The invention discloses an application of melatonin in improvement of absorption of sulfur by vegetables. The application particularly includes the steps of: 1) mixing the melatonin with additives to prepare a melatonin aqueous agent; and 2) spraying the melatonin aqueous agent on leaves of a target plant satisfying sulfur deficiency conditions, or applying the melatonin aqueous agent to a sulfur-deficient nutrient solution or sulfur-deficient soil in which the target plant grows. A researching result proves that after exogenous supplement of the melatonin, the absorption of sulfur by vegetables is improved and activity of a key enzyme related to sulfur metabolism is enhanced too. The melatonin promotes synthesis of a sulfur containing compound and further improves stress resistance of plants. The melatonin, especially in the situation of sulfur-deficient soil, has more significant effects.