Abstract: The test was conducted to study the influence of the different nitrogen concentration in float system on some agronomical and physiological characters of flue-cured tobacco plant at transplanting and 30 days after transplanting respectively. The results showed that: (1) The dry matter accumulation of tobacco seedlings were improved when the nitrogen concentration was increased from 50 to 450 mg/kg in float system, while that of the seedlings treated with 550 mg/kg was decreased; (2) Nitrogen concentration in float system had great influence on root activity. Root activity of the seedlings treated with 150 or 250 mg/kg were higher than others；(3) Proline content in tobacco seedlings was decreased at the time for transplanting when the nitrogen concentration was increased in float system; (4) The activity of peroxidase, catalase and polyphenoloxidase were observed highest in plants at 30 days after transplanting which were nursed in float system with nitrogen concentration at 150 mg/kg, while those of tobacco seedlings treated with more nitrogen in nutrition solution deceased. And it was reverse for the content of protein. In a word, it could be considered that the ability of resistance of HD plant should be better if the seedlings were nursed in float system with nitrogen concentration at 150 to 250mg/kg.