Abstract: The diffusion approximation solution with one-point-source and two-point-source are investigated. In this theoretical model, considering a pencil beam incident on a semi-infinity scattering medium, the source function is simplified as one-point-source satisfying the dipole moment and two-point-source satisfying both the dipole and the quadrupole moments. The spatially resolved diffuse reflectance for the simplified sources are studied, which indicate that two-point-source approximation can more describe radiance close to the source and more depict the anisotropy radiance superior to one-point-source approximation.