Considering viaduct pier and traction network’s electrical coupling, this paper focuses on the study of electromagnetic transient effect for electric multiple unit (EMU) passing split-phase insulator, which has important significant to the operation safety of EMU, since the traction lines of HSR (high-speed railway) power traction system are mostly laid on viaduct. Firstly, equivalent circuit model including traction power supply circuit and viaduct pier loop circuit is established. Secondly, this paper analyzes and solves the coupling coefficient of viaduct pier and various electrical parameters. Thirdly, the vehicle-catenary-viaduct coupling lumped parameter model regarding each transient process for EMU passing split-phase insulator is built by ATP-EMTP. Finally, the corresponding electromagnetic transient processes, which respectively consider viaduct or not, are simulated, and their influencing factors are analyzed in detail. The simulation results indicate that the viaduct looped circuit provides with effective discharge channel to weaken the over-voltage amplitude obviously. This work is supported by National Natural Science Foundation of China (No. U1434203, No. U1134205 and No. 51377136).