Abstract: In order to determine the dynamic response to different nitrogen forms in the main growing period of flue-cured tobacco, with flue-cured tobacco NC89 as experimental material, we studied the effects of different ratios of NH₄⁺:NO₃^– 0:100AN (Ammonium/Nitrate); 25:75AN; 50:50AN; 75:25AN; 100:0AN on biomass production, root growth and the contents of main nutrient elements. The results showed that compared with other treatments, different growth period of flue-cured tobacco biomass were highest in the treatment of 50:50AN, significantly higher than 100:0AN. Compared to supply of sole NH₄⁺, all the other treatments showed a significant positive effect on total roots length and number of roots, but average root diameter did not show much difference between treatments. The nitrogen content of leaves decreased with the growth stage in the treatments of 75:25AN and 100:0AN. Except for the rosette stage, nitrogen content of leaves in the treatment of 50:50AN were the highest. When using sole nitrate, phosphorus content of leaves was significantly lower than 50:50AN, 75:25AN and 100:0AN. Potassium contents of roots in the treatments of 0:100AN and 25:75AN were significantly higher than 100:0AN. In addition, potassium contents of leaves in the treatments of 50:50AN were significantly higher than 100:0AN except at the rosette stage. Considering the effect of nitrogen form on biomass and quality of fluecured tobacco, the optimum ratio of ammonium and nitrate was 50:50.