Optical diffuse spectroscopy is crucial in quick and non-invasive measurement of biological tissues. However, the lack of exact analytical solution limits its applications. A semi-empirical formula of diffuse reflectance measured via small apertures is elaborated on the basis of Monte Carlo method, and a function relation between the reflectivity and the aperture, relative refractive index is created. Based on this semi-empirical formula, the inversion of optical parameters of turbid biological media is performed and its application range is analyzed. Compared to the expression of diffusion approximation, the proposed analytical formula is mathematically simple and suitable for the measurement of radiation field near the light source.