Due to strong scattering, most optical tomography techniques can’t achieve practical imaging depth in turbid media such as bio-tissues. In this paper a new inner-light-source model was proposed and Monte Carlo method was used to analyze related problems. The distribution of backscattered photon number against the number of their scattering events was studied, and various illumination geometries and detecting methods were simulated. The results show that instead of rashly assuming that the number of photons reemitted from the tissue monotomically increases or decreases as scattering events accumulates; the backscattered photon number actually increases firstly and decreases later. The peak position, peak value and curve shape depend on the illumination geometry, monitoring method and tissue optical parameters.