Abstract: With the application of semiconductor switch in power systems,wideband modeling of related devices is necessary to analyze the electromagnetic interference. This paper presents an efficient method to model the passive devices with traditional equivalent circuit combined with black-box theory. Black-box impedance was obtained by the subtraction operation between the measured and the traditional model. Vector fitting was used to approximate this impedance and circuit synthesis theory was applied to provide equivalent circuits. Based on these,wideband models of water-cooled resistor,anode reactor and capacitor were presented. The results show that this new method enables the designers to retain their existing physical models while providing a means to capture the high frequency effects accurately.