we proposed a model for reflectance of a medium in ink layer. And the scattering coefficient is more complicated than the refractive index and absorption coefficient due to its complicated scattering laws and multi-direction, and the multi-direction makes different scatting coefficients in different situation. The goal of the work proposed in this dissertation is to develop a new method to analysis the scattering situation using random walk. By analysis the probability of the scattering of the particle, we obtain the reflectance factors of the diffusing medium with the observation geometry. The present model enables one to predict the reflection spectrum of a particle medium, then to estimate the color and brightness.