The assumption that biological tissues are semi-infinite in the longitudinal direction and infinite in the transverse direction is applied in traditional diffusion equations. Aiming at the biological tissues, e.g. forearms and fingers, that are not infinite in the transverse direction, a diffusion model for random multi- layer rectangular biological tissues is established. It is assumed that biological tissues are semi-infinite and multi-layered in the longitudinal direction, and are rectangular in the transverse direction. Under the rectangular boundary conditions, exact solution of the diffusion equation for light in semi-infinite multi-layer rectangular media in steady-state is provided based on the diffusion equation of light propagating in biological media and by combining with the extrapolated boundary conditions. The spatially resolved diffusion is calculated using the established model, and corresponding Monte Carlo simulation is programmed to demonstrate correctness of the equation. The established equation does not only solve the problem of rectangular media in the transverse direction, but also the problem of media that are infinite in the transverse direction and semi-infinite in the longitudinal direction, even the problem of tissues that are infinite in the transverse x or y axial direction and finite in the other axial direction.