Previous studies have demonstrated that the techniques of polarization and oblique incidence can effectively suppress the scattered light from deep layer, thus providing feasible methods to detect optical properties of superficial tissue. Monte Carlo simulation is used to study their mechanism. The effects of optical properties and incident angle on detection depth are investigated respectively. It is found that anisotropy factor and incident angle affects the detection depth much. While for the cases of small anisotropy (<0.8) or large incident angle (>1.4 rad), detection depth can be constrained in the depth of less than 2 fold of mean free path. The techniques of polarization and oblique incidence can effectively reduce scattering times and penetration depth of detected photons, which indicates that it can effectively differentiate signal from superficial tissue and remove background signal from underlying tissue through combining these two techniques.