This paper studied the LIAF transportation mechanism in tissue by a series of Monte-Carlo simulations, and investigated the effects of boundary condition, optical properties, structure, and the quantum yield of tissue chromophores on LIAF spectra. According to the layered structure of gastrointestinal tract and the penetration depth of laser beam, a bi-layer tissue model was set up for simulating the distribution of LIAF spectra obtained from normal tissue and adenoma. The results showed that an index-mismatched boundary reduced both the amount of excitation and emission that are remitted from the medium surface. In addition, the origin of fluorescence in normal and cancerous tissues was discussed. We found out that the cancerous mucosa has significant lower fluorescence efficiency compared to normal tissues. The reason is two-fold: firstly, the fluorescence quantum efficiency ofcancerous mucosa, which is the percentage of fluorescence of the cancerous tissues that get excited, is lower than that of normal mucosa., secondly, due to the higher absorption coefficient of cancerous mucosa, the amount of excitation light reaching submucosa layer is reduced, resulting in significantly lower remitted fluorescence from submucosa layer.