A modified delta-P1 approximation is studied in this paper. An analytic solution Rδ-P1(ρ) of this model for reflectance in double source approximation is developed by taking the source series expansion terms to the first order. It is shown that when the source in extended to first order, a better description for high absorption and high scattering media will be developed. The new model can give satisfactory results in describing irradiance distribution close to source compared with the RSDA(ρ) of diffusion approximation. Moreover, Rδ-P1(ρ) provides the prediction of the second-order parameter γ, which is significant for developing the inversion algorithm to recover optical parameters from spatially-resolved diffuse reflectance by using smaller source-detector separations.