Abstract: To explore the effects of seed priming with citric acid on tomato seed germination, seedling growth and physiological characteristics under low phosphorus stress, 'Zhongza 9' was cultured in sand at four phosphorus concentrations: control (0.66 mmol·L^-1), mild phosphorus deficiency (0.44 mmol·L^-1), severe phosphorus deficiency (0.22 mmol·L^-1), and non-phosphorus (0 mmol·L^-1). The tomato seeds were soaked in citric acid (7.5 mmol·L^-1), and the seed germination, seedlings growth and physiological characteristics were examined. This experiment aimed to clarify the physiological mechanisms of citric acid seed priming in alleviating low phosphorus stress in tomatoes. The results showed that, compared with seed soaking in distilled water, the different concentrations of citric acid seed soaking significantly increased the germination rate, germination potential, and germination index; the optimal concentration was 7.5 mmol·L^-1. Compared with the control, tomato seedling growth under low phosphorus stress was severely inhibited; the malondialdehyde (MDA) and reactive oxygen species ( \textO_2^\bar \cdot , H₂O₂) contents significantly increased; and the superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activities significantly decreased. The soluble protein, soluble sugar, and betaine contents significantly increased, and the osmotic potential was low. The seed priming with citric acid significantly promoted tomato seedling growth, increased biomass accumulation, increased the activities of antioxidant enzymes, reduced the accumulation of MDA and reactive oxygen species, and reduced the oxidative stress in tomato seedlings. It also significantly improved the organic acid and soluble sugar contents, alleviated osmotic potential decline, increased the phosphorus content, and lessened the symptoms of low phosphorus stress. In summary, priming tomato seeds with citric acid alleviated the oxidative damage caused by low phosphorus, improved the absorption efficiency of phosphorus, maintained normal growth and metabolism, and enhanced the resistance of tomato seedlings to low phosphorus stress.