A new control parameter is introduced to phase function to modify the general diffusion approximation for small source-detector separation of tissue. The results of standard diffusion approximation, general diffusion approximation and modified general diffusion approximation are compared with the Monte-Carlo simulation in steady state diffusion equation for slab boundary condition. The result shows that the modified general diffusion approximation with an appropriate control parameter is more closed to Monte-Carlo simulation for local tissue, and describes exactly the photon propagation of small separation between source and detector. The effects of system parameters, such as numerical aperture and diameter of laser on the control parameter are also discussed. This study is useful for the measurement of optical properties of tissue with high resolution.