The design of electromagnetic interference (EMI) filters needs to fulfill the EMI standards. Designing a filter is a time-consuming process for new engineers as well as for those experienced engineers. This paper measures and compares the noise spectrum of the wireless base station power prototype with and without the original filter. The ideal insertion loss (IL) of the original filter is obtained by combining calculation and simulation. It is pointed out that the effect of the original filter is not good. Based on the improved insertion-loss method, the source impedance model of the prototype is established by combining measurement and theory. A procedure for designing EMI filters for switch power supply will be presented. The filter design procedure makes it possible to design filters quickly and easily. Finally, the proposed filter design method is proved to be effective by the EMI measurement of the wireless base station power supply prototype.