The responses of canopy spectral reflectance, optimum spectral indices and monitoring models for leaf nitrogen accumulation (LNA) in wheat to the changes in bandwidths were analyzed on the basis of the sensitive bands of 720nm, 860nm and 990nm. The results showed that with increasing bandwidth, the reflectance increased sharply at 720nm and slowly at 990nm, but decreased at 860nm. The values of NDSI(R860, R720) and RSI(R990, R720) decreased gradually, along with reduced accuracy of calibration and validation for monitoring models. The monitoring models based on NDSI(R860, R720) displayed stable performance with the bandwidths of the key bands less than 96nm and 28nm, respectively, while those of RSI(R990, R720) less than 3nm and 13nm, respectively. These results helped to provide theoretical basis and technical approach for manufacturing economical portable nitrogen monitoring instrument.