Abstract: To harden electronic systems against powerful electromagnetic pulse, a hybrid method which combined finite difference time domain (FDTD) and Simulation Program with Integrated Circuit Emphasis (SPICE) is provided. The effect mechanism of powerful electromagnetic pulse interaction with microstrip line is studied. The results show that the coupling of EMP on microstrip line has two different modes related to frequency, due to the complex structure of microstrip line. One is the surface wave coupling whose polarity is along the strip line, and the other is the slot coupling whose polarity is vertical to the strip line. The influence factors of transfer function under two coupling modes are very different. The study is significant to improving the defense ability of electronic system against EMP effects．