Abstract: In order to investigate the response of dry weight and morphological characteristics of flue-cured tobacco root system to drip nitrogen fertigation,10 drip nitrogen fertigation treatments of different nitrogen ratios 0,28,42 days after transplanting were designed using the cultivar K326 in the frame conditions.Root dry weight,root-shoot ratio and root morphological indexes were assayed.The results showed that root dry weight,root-shoot ratio,root surface area at 15-25 cm in the vertical direction and 15-20 cm in the horizontal direction from the infusion point decreased with the increased rate of nitrogen topdressing.Root dry weight and root-shoot ratio of the T2 treatment (50% of the N fertilizer applied at transplanting plus 2 times of drip fertigation) were 27.08%,3.61% and 13.79% higher than T1(50% N applied at transplanting plus 1 drip fertigation),respectively.When 30% and 10% of the nitrogen fertilizer was applied at transplanting,two times of nitrogen topdressing caused a significant decrease in root dry weight,and an increase in root surface area at 20-25 cm in the vertical direction and 15-20 cm in the horizontal direction from the infusion point.In other words,to some extent,promoted the growth of root system which was deep in the soil and far from the plants.The 10 treatments had no significant effects on the average root diameter.Under the conditions of this experiment,appropriately increasing the ratio of nitrogen at the early growth stages can promote the growth of tobacco root system.Root growth can be further promoted by appropriately increasing the ratio of nitrogen topdressing,under the premise that the nitrogen at the root extending stage was adequate.