In the present study, alfalfa canopy reflectance was researched at alfalfa squaring period under different irrigation amount at the hutubi county grassland ecological station. Determining the spectral diagnostic model of alfalfa leaf moisture content was determined by spectrometry. The results showed that (1) The spectral reflectance of alfalfa canopy gradually decreases with the increase in the leaf water content in the near infrared. (2) The spectral inversion model of alfalfa leaf moisture content established by normalized reflectance spectra is superior to the original reflectance spectra, and the prediction model established in the 1 344~1 660 nm band has the lowest average relative error (7.8%). (3) In this study, the spectral diagnostic model of the leaf moisture content is: Y＝0.962-7.560X1 451+5.295X1 473. The spectral prediction model of the alfalfa leaf moisture content can provide a basis for decision making for scientific irrigation of alfalfa.