In order to extract spectral feature for achieving spectral classification, 16 multispectral images of pseudoperonospora cubensis for cucumber disease, including visual light band, near-infrared band and a panchromatic band, were captured for 6 samples in this paper. Then, the distinguish ability of all multispectral images were computed by average gray, standard error and background gray. We ordered the 16 bands using the distinguish ability to 6 samples. The bands with high distinguish ability were extracted as spectral feature bands of cucumber′ pseudoperonospora cubensis. The result showed that it was speedy to extract spectral feature of cucumber disease by the gray of images. The multispectral images of 700, 600, 589 and 546 nm bands have plentiful black-white distribution and good contrast. They were spectral feature bands of cucumber′ pseudoperonospora cubensis.