Rare earth is physiologically active elements，which can affect the growth and development of plants，and plant roots can respond to exogenous plant growth regulators and regulate plant growth.Using tomato seedings as materials，the effects of lanthanum and cerium on the growth and root exudates of tomato seedlings were analyzed by hydroponic experiment. The results showed that the application concentration of rare earth elements at 0.05 mmol/kg promoted the increase of tomato plant height and leaf area，but inhibit them when the concentration reached 0.1 mmol/kg.The application of rare earth elements inhibited the growth of plant roots.The components and contents of tomato root exudates were identified and analyzed.There were 110 metabolites with significant difference between rare earth treatment and blank group，belonging to 10 secondary classifications and 38 tertiary classifications，among which the differences among Fatty acyls class ification，Carboxylic acids and derivatives class ification and OrgaNO_xygen compounds class ification were the most significant.However，the effect mechanism of various root exudates on tomato growth still needs to be further studied.