Abstract: In order to study the time-domain response characteristics of terminal voltage/current for complex wiring harness system (multi-conductor, nonlinear load, dielectric coatings) under external field illumination, a field coupling to transmission line modeling method based on the Baum-Liu-Tesche(BLT) equation is proposed in this paper. Through re-formulating the multi-conductor BLT equation in time domain, the response of nonlinear load connected to terminal of the wiring harness is solved, which preserves the advantages of high computing efficiency and applicability to line networks of classic frequency domain BLT method. By introducing the coating coefficient and modifying the source vector of BLT equation, the modeling error caused by ignoring the distortion effect of dielectric coatings on incident field is eased. Finally, the calculation results are compared with the results obtained by finite difference time domain and finite integer technique, which verifies the accuracy of the model in dealing with the field coupling to complex wiring harness problem.