Abstract: By studying the effect of Populus alba var. pyramidalis plant height and protective enzyme activity under foliage spray and determine the optimal spraying concentration of nitrogen fertilizer and provide a theoretical basis for the reasonable application of nitrogen fertilizer production. Test used 1 years old cuttings of Populu alba var. pyramidalis seedlings as materials. Concentrations of nitrogen fertilizer spraying as 0%(T1，CK), 0.70%(T2), 1.40%(T3), 2.10%(T4), 2.80%(T5) plot experiment design and three replications. The results showed that: Populus alba var. pyramidalis plant height showing increased with the increase concentration of nitrogen fertilizer spraying and between T4 and T5 treatment was no significant difference and significantly higher than control. T4 treatment POD activity as the highest value in July, August, October and increased than control 162.49%, 69.59%, 302.24%. T4 treatment SOD activity increased than control 21.36%, 25.78%, 17.29% in august to October; PPO enzyme activity during the growing trend exhibit has been reduced and T4 treatment as the highest value in July, August, October and increased than control 87.02%, 52.55%, 124.54%. Comprehensive analysis that Populus alba var. pyramidalis seedling nitrogen spray concentration of 2.10% best.