In EMS (electromagnetic susceptibility) simulation of electronic devices, the mathematical modeling can be adopted to conduct systematic analysis of EMS and optimization of the filter design. This paper has presented one modeling method for the EMS simulation analysis of electronic devices, based on the system identification supported by experimental data. A mathematical modeling of the power module in the relay under surge interference has also been constructed according to the input and output response of this module measured by oscillograph. Good agreements between the SPICE simulation and the measurements demonstrate the validity of this modeling method