Abstract: Time-domain analysis of the coupling of the field to transmission line plays a important role in electromagnetic pulse protection of shielded cables. A digital filtering fitting method is proposed to avoid the complicated convolution between the ground impedance and the cable induced current. The adopted soil impedance model and the basic theory for this digital filtering fitting and the calculation result of this method are introduced. By the digital filtering fitting method, the complex convolution is avoided. This method can be used in the analysis of the field to transmission line radiated by the non-uniform field or connected to nonlinear loads.