Quantitative description of the tongue proper and tongue coat isperceivedas the important content for tongue diagnosis in traditional Chinese medicine, which directly affects the accuracy of clinical diagnosis. A new method fortongue proper and tongue coat separationusing spectrum dissimilarity index (SDI) by double-wavelength ratio is presented, which isbased on hyper spectroscopy and theory of biomedical photonics. The 343 wavelengths of hyperspectral tongue information of subjects is obtained in the range of 371.200 to 992.956 nm. Moreover, the hyperspectral data of 15 subjects is selected randomly for spectral characteristics analysis. By comparison with the spectral curve of the wavelength range, it can be seen that optical spectrum characteristic dissimilarity between tongue proper and tongue coat is resemble the optical characteristic of hemoglobin, since the surface of the tongue is full of blood vessels and the absorption of water is small in the range of 500 to 750 nm. In addition, double wavelength optic reflection ratio is used to extract optical spectrum characteristic dissimilarity index of tongue proper and tongue coat based on spectral absorption features of hemoglobin atthe wavelength of 573 and 700 nm, finally tongue proper and tongue coat is separated. It is can be seen that the SDI with tongue proper and tongue coat of one subject is a certain difference and has the obvious difference index line between tongue proper and tongue coat. The experiment results show that spectrum dissimilarity indexis very helpful for tongue coat and tongue proper separation in traditional Chinese medicine, and this new analysis approach may providean objective standard for TCM tongue diagnosis.