As to most methods of detecting the inner information of inhomogenous tissue, a significant issue is that the detection position is ambiguous because of the complexity of human tissue structure and discrepancies among individuals. This paper studies the best source-detector distance (SDSbest) to detect internal information of a fat-muscle tissue with spatially resolved diffuse reflectance spectra. In order to weaken the measurement error caused by the discrepancies among individuals and multiple backscattered photons, and according to the transmission model of light in complex biological tissue, then we added the constraint condition——two ideal “banana shape” paths——to define the effective photon ratio(SNR), which was used to select the best source-detector separations (SDSbest), and the results from Monte Carlo simulation modified by adding constraint condition were statistically analyzed, and we regard the SNR as a basis and analyze the relationship between the fat thickness (hf), the absorption coefficient of a fat layer (μaf), the absorption coefficient of a muscle layer (μam) and the source-detector distance (SDS), and hf is used as the independent variable to develop a linear regression model to predict SDSbest. The result showed that μaf and μam have no effect on SDSbest when 0