A three-layered model of Monte Carlo simulation for photon flux in skin was presented. From the results of simulation we know that by varying the distance between the inner edge of the ring detector and the beam, we can measured the blood flow at different depths. The dependence of detected photon number on the frequency shift showed an exponetial decrease. By varying the blood volume fraction (Vb) , we investigated the effect of multiple scattering on the total frequency shift and the mean frequency shift. When Vb<0.01, the mean frequency shift was a constant and it reflected the mean velocity of red blood cells, while Vb>0.01, it reflected the root-mean-square velocity.