Abstract: To investigate the influence of exogenous melatonin on the physiological characteristics of flue-cured tobacco at fast-growing stage under drought stress, a pot experiment was conducted using the flue-cured tobacco cultivar K326 in southwestern tobacco growing areas. The physiological regulatory effects of root irrigation and foliar application of melatonin at different concentrations (0, 0.05, 0.10, 0.15, 0.20, 0.40 mmol/L) on K326 under drought stress were investigated, and the indicators that could alleviate drought stress on K326 were screened using principal component analysis. Meanwhile, a comprehensive evaluation was carried out using the membership function method. The results indicated that drought stress caused a decrease in the biomass of cured tobacco and significantly inhibited plant growth. Exogenous melatonin effectively increased biomass under drought stress, increased relative leaf water content (RWC), chlorophyll content, superoxide dismutase (SOD) activity, peroxidase (POD) activity, catalase (CAT) activity, proline (Pro) and soluble protein (SP) content, while decreasing leaf malondialdehyde (MDA) content and superoxide anion (O₂^-) production rate of the flue-cured tobacco. Principal component analysis revealed that antioxidant factors had the highest proportion among the evaluated indicators for alleviation of drought stress by melatonin, followed by photosynthetic factors and osmotic regulation factors. Comprehensive evaluation using the membership function method showed that foliar application of 0.10 mmol/L melatonin solution gave the best results. Therefore, the application of exogenous melatonin can potentially alleviate the inhibitory effects of drought stress on the growth of flue-cured tobacco at the fast-growing stage and enhance its adaptability to drought conditions.