Abstract: The impedance modeling of the aircraft three-stage starter/generator is very crucial for conduct EMI analysis, and multi-EMI sources are included in this system. In this paper, a universal wide frequency equivalent circuit model was proposed which considers the stator impedance, rotor impedance and the mutual coupling effect comprehensively. A 'black box' model based multi-port network theory was applied to replace the machine, thus the Z matrix could be used to describe the frequency character of the machine. The extraction and calculation processes were also discussed in this paper. The theoretical impedance error caused by the coupling neglecting was deduced, then a general wide frequency circuit model was presented. The circuit model was parameterized based on vector fitting technique. The proposed method could be used for EMI analysis of the aircraft three-stage starter/generator, and it had the advantages of high precision, low complexity, more flexible.