The most desired aim of clinical detection is to minimize the dimension of the detection instruments but still maintain high resolution and high precision. One of the most important mechanisms for maintaining the life of human beings is the human circulatory system. Thus, this researches focus on a non-invasive technique of photon measuring. From the measured result we could obtain important biomedical parameters for pathological diagnosis. This project is to implement a non-invasive optical flow velocimetry for detecting the human circulatory system under the skin surface. Then numerical simulation and model analysis of the transmission of photon on the skin surface is calculated via Monte Carlo Method. The source of the incidence photon is He-Ne laser. The signal is transmitted and detected via Y-type optical fiber. Optical heterodyning is used to measure the frequency difference between the reflection wave and the original incidence laser wave. This is a real-time and continuous detecting blood flow velocimetry. We find that this is a reliable tool for doctors when doing clinical diagnosis.