Abstract: Aiming at the transient response of the passivity of multiconductor transmission lines（MTLs），a transient response model based on lumped equivalent sources is proposed. By researching the solution of telegrapher equation of the MTLs exposed to incident field in the frequency domain，the distributed voltage and current sources excited by incident field were decoupled with the exponential MTLs matrix. The lumped equivalent voltage and current sources model were built. The recursive formulation of lumped equivalent in time domain was derived to avoid complex inverse Fourier transform and convolution calculation. Then the discrete recursive equations，including linear loads，nonlinear loads and unequal length MTLs exposed to incident field，were built based on finite difference time domain method. The method proposed in this paper is verified effectively by comparing the simulation results of lossless MTLs. At last，the transient responses of MTLs with linear and nonlinear loads，as well as unequal length MTLs exposed to incident field were analyzed by experiments and simulations.