Based on a theoretical model for spatial diffusion reflection with two-point-source hybrid diffusion approximate to a semi-infinite homogeneous turbid medium, expressions of the sensitivities for the absorption coefficient μa and the reduced scattering coefficient μ's are derived, and the effect of the sensitivities of the two optical parameters μa and μ's on diffuse reflectance is studied systematically in small volumes of highly absorbing medium. The results show that the sensitivity of the absorption coefficient increases approximately linearly with the increase of source-detector separation ρ, with a slope proportional to 1.4 (μ's/μa)1/4, and also indicate an optimal probe spacing ρopt at which a variable reduced scattering coefficient has a smallest effect on the measurment of optical absorption in the turbid medium. The value of ρopt is approximately equal to 3.4 times transport mean free path. The study is of great significance for the measurment of the optical properties of biological tissues and the application of diffuse reflecance spectroscopy technology.