There are two main ways to couple high altitude nuclear electromagnetic pulse (HEMP) impact on electronic equipment. On the one hand, it is the “front door coupling” mode that the electromagnetic interference enters the electronic system through the antenna coupling channel on the equipment (product); and on the other hand, it is the “back door coupling”, that is, the coupling is carried out through the gaps and holes of the shell, power cord, cable and chassis on the equipment (product). In this paper, the coupling effect of electrical wiring interconnection system (EWIS) harnessing assembly against high altitude nuclear electromagnetic impact is mainly studied. By studying the characteristics and energy distribution of HEMP interference, the mathematical model of HEMP is established, the coupling effect is simulated by CST software, and tests is carried out at the same time, then the general conclusions and laws of HEMP coupling effect are obtained by synthesizing the simulation results.