Abstract: In order to explore the effect of cadmium stress on tobacco growth and the mitigation effect of exogenous melatonin (MT) on cadmium toxicity, the effects of different concentrations of cadmium on agronomic traits, SPAD values and cadmium content in leaves of Yunyan 100 seedlings were studied by hydroponic experiments, and the alleviating effects of different concentrations of exogenous MT (10, 50, 100, 200 μmol/L) pretreatments on tobacco toxicity under 100 μmol/L CdCl₂stress in tobacco were studied. The results showed that cadmium (50-200 μmol/L) could significantly inhibit the growth and development of tobacco seedlings, and exogenous MT could significantly alleviate the toxicity of cadmium in tobacco. Compared with the cadmium (100 μmol/L) treatment alone, the root length, fresh mass, maximum leaf length, maximum leaf width, plant height and SPAD values under the 200 μmol/L melatonin treatment increased by 17.50%, 131.89%, 70.83%, 51.79%, 78.43% and 87.20%, respectively. MDA and intracellular cadmium content decreased by 42.67% and 57.4%, respectively. The activities of superoxide dismutase (SOD), peroxidase (POD), glutathione peroxidase (GSH) and the content of soluble protein (SP) increased by 391.20%, 3305.57%, 332.11% and 88.52%, respectively. In conclusion, exogenous melatonin can reduce cadmium enrichment in leaves, improve antioxidant enzyme activities and cytosmotic pressure regulators of tobacco seedlings, and improve the stress resistance of tobacco plants, so as to alleviate Cd toxicity in tobacco.