Prediction of light distribution in random medium is still an unsolved problem. Although the Monte Carlo method can deal with medium with inhomogeneous and anisotropic optical properties, the long calculating time makes it hard to be applied clinically. This paper presents a block method, by which the target medium is divided into many small cubic blocks. The absorptances and transmittances of these small cubes are pre-calculated by the Monte Carlo method. The light propagation in the random medium is simulated by calculating the light transmission between these small cubes. With this method, the distribution of light absorption in target medium can be determined within one minute even in a personal computer. As an example, the distribution of light absorption in a tumor surrounded by normal tissue is shown.