The complete pathological tooth model and the criterion of caries lesion extent are established.With the programme of Monte Carlo simulation algorithm,the process of fluorescence excitation and transmission in tooth tissue is completely simulated and the relationship between the changes of optical parameters caused by the tooth tissue lesions and fluorescence distribution of QLF ia also explored.And,the legitimacy of the process above is verified using fluorescent functional of tracepro software.With the increase of lesion extent in dental caries part,the results show that the difference of fluorescence distribution between healthy and diseased tissue are increased with the increase of scattering coefficient and the reduce of absorption coefficient and fluorescence quantum efficiency.