Abstract: In order to study the nonlinear effect of shielded cables caused by radiation coupling of electromagnetic pulse (EMP), a measurement method for the surface transfer impedance under bulk current injection is proposed. The measurement system is built, then the transfer impedance is measured by injecting currents of different amplitudes, and the internal wire responses of two kinds of shielded cables are observed. The experiment shows that when the injected peak current value exceeds 2 kA, the coupling voltage nonlinear effect occurs in the multi-conductor cable which contains a ferromagnetic shield layer. The explanation of this phenomenon is also analyzed.