Abstract: Aimed to provide the theory gist for monitoring nutrient stress at different stages and assessing yield, a long term fixed fertilization experiment was conducted to select wavelengths and growth stages which were sensitive to nitrogen, phosphorus, potassium and yield in Huang-Huai-Hai area where winter wheat and summer maize crop were rotated. The canopy spectral reflectance at typical growth stages, N, P, K content in stalk and grain, and yield at harvest stage under different nutrients levels was analyzed. The results showed that canopy spectral reflectance were increase at visible light and decrease at infrared scope under nutrient-deficient state. N, K in both grain and stalk and yield was negatively correlated to the reflectance of visible light(expect for 460nm), but positively correlated to between infrared scope(760-1300 nm). The correlation between P content in stalk and crop spectral was reversed to that of N, P and yield. P content in grains has no correlation to crop spectral. On the whole, wavelengths at 560 and 810 nm were sensitive to crop N content at harvest stage and anthesis was the critical stage for N diagnosis.Wavelengths at 680 and 810 nm were sensitive to crop K content at harvest stage, and jointing was the critical stage for K diagnosis. Yield was significantly correlated to spectral reflectance throughout the stages, in which at jointing correlation was the most significant. Therefore, under the premise of confirming the dominant deficit nutrient, it is feasible using canopy spectral reflectance to monitor N, K levels and assessing yield.