The light propagations in human lung and three-layer artery are simulated by Monte Carlo method. It results that, with the increasing of the refractive index, the diffuse reflectance R_d is linearly decreased and absorptance A is almost linearly increased. But for multi-layer tissues, the influence of the refractive index in the deeper layer of the tissue on R_d is less, and the influence of the refractive index in the different layer of the tissue on A is different because each layer has its own thickness and optical property. Therefore, the refractive index is a very important parameter in bio-tissue.