A pot experiment was conducted to study the effects of different concentrations of melatonin on the growth and cadmium (Cd) accumulation of Brassica juncea var. megarrhiza under Cd-contaminated condition. The concentrations of 50, 100 and 150 μmol/L melatonin increased the root length and volume, plant height, tuber diameter and length of B. juncea var. megarrhiza seedlings, and increased the biomass of roots, tubers, shoots and edible parts, while 150 μmol/L melatonin decreased these items. Different concentrations of melatonin improved the photosynthetic pigment (chlorophyll a, chlorophyll b, total chlorophyll and carotenoid) contents and antioxidant enzyme (SOD, POD and CAT) activities of B. juncea var. megarrhiza seedlings, while reduced Cd contents in roots, tubers, shoots and edible parts of plant seedlings. With the increase of melatonin concentration, Cd contents in the different organs of B. juncea var. megarrhiza seedlings showed the reducing trend. Therefore, the spraying of melatonin could promote the growth of B. juncea var. megarrhiza seedlings and reduce its accumulation of cadmium.