Abstract: In this paper, the coupling and penetration of electromagnetic pulse through lapped joints in conducting screen are simulated by the finite difference time domain (FDTD) method. Equivalent electromagnetic current sheets on the aperture and the transmission field are computed. For obtaining the far zone transmission field easily, the radiation of equivalent electromagnetic current sheets in a quarter-space is replaced by the radiation of equivalent electromagnetic current sheets and their images in free space based on the equivalence principle. This equivalent scheme avoids the difficulty of computing the Green’s function with the boundaries of lapped joints. During extrapolating, sub-area extrapolation is applied which saves much computing time. Finally, by analyzing the variation of coup