In many cases, EMI filter plays a key role in EMC design. However, the performance of EMI filters, especially in high frequency, is often different from previous estimates. Based on this situation, this paper first theoretically calculates the performance of EMI filter, and then analyzes the effect of the parasitic parameters of each component of the filter and its near-field coupling on the filter performance. Based on these analyses, high frequency equivalent model of the first-order π filter is completed, which verifies the rationality of the analysis.