A new caries detection method of fluorescence spectrum which contains backscattering, back-reflected and auto-fluorescence is proposed after researching on the interaction mechanism of laser with dental tissue and relationship between remitted light and optical properties of the tissue. Not only auto-fluorescence but also back-reflected and back-scattering were produced in the process of the interaction. All of these lights contained rich information related with caries components, structure, spatial conformation, and so on. In order to obtain the relationship between those information and various stages of dental caries, an experiment of caries in vitro was designed and carried out. Investigation was performed on 104 samples by using He-Ne laser (λ=632.8 nm,2.0±0.1 mW) as a source of probing radiation. Using the Rayleigh scattering to normalize the intensity of measured spectrum, we defined anti-Stoke′s scattering normalization coefficient C1, which can describe morphological changes in dental tissue, and Stoke′s fluorescence normalization coefficient C2, which can describe chemical changes in tooth, and then, we evaluate various stages of dental caries by analyzing two normalization coefficient of caries characteristic spectrum. Preliminary results showed a high potential of using this new method to diagnose various stages of dental caries, and it is more sensitive to white caries, filling and Dental calculus.