Light transmission in tissue is a random process of absorption and scattering of diffuse optical, and expressed by the radiative transfer equation. Because the equation is complex, it can"t get the analytic solution directly, the traditional method is the Monte Carlo simulation. The paper proposed a modified method in step and weight of traditional Monte Carlo simulation, and complete the photons transmission process in tissue quickly. With the same simulation conditions, the experiment is simulated by the Monte Carlo method and improved method, and it shown the improved method can greatly improve the simulation speed; Then the simulation results with heterogeneous medium shown that the proposed method is effectiveness, not only improve the speed, but also ensure the accuracy.