This paper presents an efficient time-domain macromodeling algorithm to calculate current and voltage responses of overhead and buried lines to incident field coupling. Based on transmission line theory, the proposed macromodel adopts the analog behavioral modeling of Spice solvers and generalized Method of Characteristics (MoC) to model the frequency-dependent variables, and to calculate the convolution in time domain. This method has a wide applicability as it can both model field coupling to overhead and buried lines. Compared with the finite different time domain method, there is no need to discretize time and space, and adopt numerical inverse Fourier transform or vector fitting method to obtain transient parameters. The efficiency of the macromodel is not limited by line length, so it is valid for modeling long multiconductor lines. Furthermore, the environment and characteristics of HEMP are studied in time and frequency domain, respectively. Finally, two examples are studied to validate the proposed method for field coupling to overhead and buried lines. Using this method, effects of overhead grounding lines on transient response of three-phase power lines and buried power cables terminated with linear or nonlinear protective loads are investigated, respectively. All results show that the macromodel can efficiently calculate the transient responses of incident field coupling to overhead and buried power lines in time domain, especially for long multiconductors with nonlinear elements.