A model called imposed excitation method for targets on the ground in time domain is described.The method comprises two steps：in the first step the scattering of the ground is calculated;in the second step the scattering of targets is calculated by imposing the scattering wave got at the first step upon the targets.A truck is involved in this paper.The scattering waves and RCS in VHF/UHF frequency are calculated.Good agreement between calculated and experimental results is obtained.It shows the effectiveness of the method.