Aiming at the nearly isotropic optical radiation and source-detector separation that is much longer than one transport mean free path in the standard diffusion approximation model, an improved model is developed by adding a point source at the air-biological tissue boundary to minimize the error close to the light source. The effect of anisotropy and the ratio of μ′s/μa on diffuse distribution is analyzed, where μ′s is the effective scattering coefficient and μa is the absorption coefficient. The inversions of the optical parameters μa and μ′s are solved by using the improved model combining the nonlinear least squares method. The results show that compared with the standard diffusion approximation model, the improved model can reduce the error of diffuse distribution curve significantly and improve the inversion accuracy. The improved model provides similar inversion accuracy to the two-point source Delta-P1 approximation model and has no poor reproducibility of inversion parameters exhibited by the latter.