With a mobile spectral radiometer, spectral reflectance of wheat canopy was observed continuously. And analysis was done of spectral response from the canopy of wheat variant in P fertilization rate at various developmental stages and relationship between spectral reflectance and wheat yield. Statistic technologies such as T-test were used to search for sensitive bands of wheat canopy reflectance responding to different P fertilization levels and hence a key developmental stage for determining whether or not P fertilization is reasonable. The results show that NIR (810~1100 nm) exhibited much better ability to differentiate variant P fertilizer levels than VIS, especially during the stage from jointing to booting, so that NIR (810, 870, 950 and 1100 nm) could be defined as sensitive bands to P nutrition of wheat and the stage from jointing to booting could be determined to be the key developmental stage to diagnose wheat P nutrition by means of canopy reflectance. Thus a good regression equation can be built up between NDVI and wheat yield.