Field experiments were carried out on two cultivars of cotton at different growth stages under different nitrogen levels using portable active remote sensing spectrometer Greenseeker to study the effects of different nitrogen application rate on cotton canopy NDVI,nitrogen accumulation rate and leaf nitrogen content.Diagnostic methods for crop nitrogen nutritional status which was simple,rapid,non-contact under field conditions were also studied.The result indicated that canopy NDVI showed in a single peak curve change at cotton growth stage,and peaked at around 90 days after emergence and then gradually reduced.Canopy NDVI increased with increasing nitrogen fertilizer in proper range of nitrogen level，but when nitrogen level was higher than N2,the NDVI was not affected significantly with the increasing of nitrogen level.NDVI was positively correlated with nitrogen accumulate rate and leaf nitrogen contents.The coefficients of determination for the stepwise regression were 0.844 7 and 0.798 0 during flowering stage.Greenseeker provides an effective means for diagnosis of nitrogen in cotton.