Abstract: This paper reports research on the effect of exogenous nitrogen on the root characteristics of “Gannong No. 3” Alfalfa at different growth stages. Samples were cultivated for the whole growth period using the sand culture method under the condition of complete nutrient solution. Three exogenous nitrogen forms (nitrate nitrogen, ammonium nitrogen and nitrate nitrogen) were investigated; with the ammonium nitrogen solution at 1∶1 and the others are three solution levels (CK, 105, 210 mg/L). Results showed that under different forms of nitrogen treatment, the root biomass, root surface area, root volume, root activity, root nodule number, nodule weight, and nitrogenase activity of alfalfa are significantly higher than that of CK. With increasing nitrogen levels, there were increases in the proportions of alfalfa root biomass, root surface area, root volume, root activity, root nodule number, nodule weight and nitrogenase activity. The effects are in the following order: NO₃^- -N and NH₄⁺ -N mixed culture the highest, followed by NH₄⁺ -N and with NO₃^--N the lowest. The impact on root average diameter was not large. The maximum values were reached by NO₃^-+N H₄⁺ with a concentration of 210 mg/L. In terms of the whole growth stage, root biomass, root surface area, root activity, root nodule number, nodule weight and nitrogenase activity were significantly different during the seedling, budding and flowering stages. However, differences during seed podding and podding were not significant.