From the similarity relations of radiative transfer theory,a parameter,namely characteristic length,was introduced on the basis of P_1 and P_3 satisfying approximatly the first-order similarity and third-order similarity relations.The relation between the characteristic length and the spatially resolved reflectance of tissue close to source was studied by using P_3 approximations expression of semi-infinite turbid medium and spatial-resolved reflectance.The research shows that:the nearest distance relates with a characteristic length charactering tissue when the measurement closing source,the width of detector can be decided by the characteristic length of tissue,and the number of detecting fiber may be estimated with the diameter of practical fiber.