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It's well known a switched-mode power supply is easily to produce strong electromagnetic radiation emission and fails in EMC test for its RFI exceeding limits between 30MHz-200MHz. Based on asymmetry line antenna theory, a novel far field common mode radiation model, including an equivalent driving source, radiation structure and some key influence factors, is identified and built up for a small flyback power supply. Radiation characteristics of this model are predicted by using Ansoft HFSS and its effectiveness is verified by experiment. In the end, the radiation role of some key factors, such as the length of output cable, CM impedance of AC grid, layout of cable and reflected ground, are studied using simulation in detail.