Abstract: To simulate the shielding of mail irradiation system, Monte Carlo method is used to investigate. EGSWIN system, developed by Department of Physics Engineering in Tsinghua University, is adopted to simulate the source. The distribution of energy, angle and intensity of the bremsstrahlung from impingement of electron beam on an Al plate is calculated and a point photon source is defined as a substitute. The calculation and comparison validate the exactness and high efficiency of EGSWIN.