RS-485 as a widely used serial communication interface, the current research mainly focuses on the level of serial communication, and the impedance characteristics of the interface circuit are seldom studied. Based on the physical characteristics of RS-485 interface, this paper establishes an equivalent circuit model under high-altitude electromagnetic pulse (HEMP) coupling by analyzing the circuit composition and coupling mechanism of the normal signal circuit of RS-485 interface circuit and the signal circuit under HEMP coupling. Moreover, the port voltage and current of the RS-485 interface under the coupling of the pulse source and HEMP are obtained respectively through experiments. Combined with the Space simulation model, the real equivalent impedance of the signal circuit under HEMP coupling is finally obtained by adjusting the values of resistance, capacitance and inductance, which provides a reference for determining the equivalent impedance of various serial communication interface circuits. Lay the foundation for subsequent protective measures.