In the electrified railway sections,the offline bow net arc caused by pantographcatenary detachment is a main EMI noise to electrifed railways. The paper discussed the mechanism of EMI noise production through the theoretical analysis of the basic form of the gas discharge at atmospheric pressure. Then the dissertation further elaborated that electron swarm oscillate,caused by near cathode effect of AC arc is the radiation source based on streamer theory. According to the electron swarm oscillate,a dipole antennas model was presented in the paper. Moreover,HFSS15 was used to simulate radiation characteristics of pantograph contact loss electric arc. The results are consistent with the experimental results reported in the literature.