A recommended nitrogen topdressing model was built up using normalized difference vegetation index (NDVI) of cotton canopy, which was obtained by active GreenSeeker sensor for diagnosing the nitrogen nutrition in cotton plant, and the precise management of nitrogen nutrition for cotton by topdressing was realized. The field test data treated with different nitrogen fertilizer levels in 2011 and 2012 were obtained, and the quantitative correlations between NDVI and fertilizer rate as well as NDVI and yield of cotton were established. Combining with the nitrogen fertilizer effect function, the best nitrogen application rate was calculated, and the nitrogenous fertilizer recommendation model based on spectra was established and verified by field test data of 2013. The results suggested that NDVI of cotton canopy increased with the increase of nitrogen fertilizer level, and then stopped increasing when nitrogen fertilizer was up to a certain level. The NDVI of cotton canopy presented a liner increasing trend with nitrogen fertilizer rate increasing in four growth stages, full bud, flowering, full boll and early wadding stages, and the corresponding thresholds of NDVI were 0.695, 0.833, 0.881 and 0.809, respectively, when the total application rate of nitrogen fertilizer was 294.7kg/hm2 during the whole growth period of cotton. In addition, as NDVI decreased by 0.001 from the thresholds, the amounts of applied nitrogen fertilizer were 0.24, 0.91, 1,11 and 0.16kg/hm2 at four different stages, respectively. Field experiment illustrated that the nitrogenous fertilizer recommendation model could reduce the fertilizer application rate under guaranteeing the cotton yield.