The scattering and absorption properties of human breast are very important for cancer diagnosis based on diffuse optical tomography (DOT). In this study, the dynamics of photon migration in three-dimensional human breast model with various source-detector separations is simulated based on a Monte Carlo algorithm. The three-dimensional human breast structure is obtained from in vivo MRI image. The breast model consists of skin, fatty tissue, glandular tissue, sternum and ribcage. The backscattered diffuse photons from each layer in breast are recorded by marking the deepest layer which every photon can reach. The experimental results indicate that the re-emitted photons contain more information from deep tissue regions with the source-detector separations because of the strong dependence to the resolution and sensitivity in DOT imaging. The geometric position of the source-detector separations were optimized in this study. The different sizes of breast tumor were modeled to analysis of optical image characterizations. Finally, the tumor images from different deep information were obtained with temporal profiles.