Abstract-A novel field-line-circuit hybrid algorithm based on finite difference time domain (FDTD) method is devoted to predicting the electromagnetic responses of transmission line with multi-ports network in a shelter in this paper. The full wave FDTD method, transmission line FDTD method, and the modified nodal analysis (MNA) are combined to be compatible with the multi-level electromagnetic (EM) coupling progress of the electromagnetic interference (EMI) problem. The proposed method divides the EM couplings among the spatial EM fields, antennas, transmission line networks, and terminal circuits in some typical electronic systems into different levels with appreciate simulation techniques used. The accuracy of the hybrid method is verified by comparing the terminal transient voltage responses of transmission lines with the results obtained by PSPICE, and good agreements are achieved. Numerical calculations are further performed to show the terminal coupling voltages and currents, and the effects of incident directions and polarizations of the illuminated electromagnetic pulse (EMP) are both taken into account.