Abstract: Based upon the fact that absorptivity of a substance at a certain wavelength is a constant value ,the abeorbance ratio at each wavelength to a specified wavelength was used as classifi- cation features.The Shannon information content for each wavelength channel was calculated for UV spectra of Dendrobium extracts,the l0 wavelength channels with the highest information content were retained as a compressed basis set for principal component analysis(PCA).The results show that PCA is more intrinsic ,convenient and feasible than microscopic method.