Abstract: Considering the return flux contamination problem in spacecraft operation, the return flux on a circular disk surface due to ambient scatter of outgassing molecules is simulated using the Test Particle Monte Carlo(TPMC) method. Results about the return flux(RFR) obtained here are in good agreement with the references. Some factors affecting the return flux are analyzed. Computational results show that the RFR increases linearly with the increase of the radius of disk surface, and the same linear relations are valid for the freestream velocity and freestream molecular number density. As the freestream angle of attack rises, the RFR increases first but decreases then. The RFR decreases and keeps a stable value with the increase of the temperature of outgassing surfaces. As the freestream molecular mass increases, the RFR rises and reaches a stable value. The RFR increases slowly with the increase of the freestream temperature. Moreover, the mechanism of how the factors affect the RFR is analyzed based on the molecular dynamic theory.