We have simulated the photon migration with various source-detector separations based on a three-dimensional Monte Carlo code. The whole brain MRI structure images are introduced in the simulation, and the brain model is more accurate then the previous studies. The brain model consists of scalp, skull, CSF layer, gray matter, and white matter. We demonstrate dynamic propagating movies under different source-detector separations. The multiple backscattered intensity from every layer of the brain model is obtained by marking the deepest layer which every photon can reach. Also the influences of an absorption target on brain cortex are revealed.