Lightning current produces lightning overvoltage on the contact net, thus posing potential dangers to the safe operation of the traction system. Due to the fact that the traditional metal oxide surge arrester (MOA) is characterized with such flaws as poor impact resistance, low discharge efficiency and potential internal leakage current, lightning arrester breakdown and catenary insulator flashover will occur when the overhead catenary is struck by lightning. In view of this problem, by adopting external series gap arresters (ESGA) protection measures, the lightning overvoltage of OCS can be limited by its gap discharge characteristics, thus ensuring the normal operation of OCS. The grounding impulse impedance of viaduct traction system is equivalent by adopting grounding grid modeling method, thus establishing the equivalent circuit of overhead section catenary system. Based on a comparison of the lightning trip rate of two different types of arresters and catenary, the simulation results show that the gap arrester is characterized with an improved protective discharge characteristics and lightning resistance characteristics, as well as a lower lightning trip rate.