The Electromagnetic (EM) scattering characteristics in terahertz (THz) band of non-uniform unstable electrical conducting target with different roughness surface is studied in this paper. The modeling concept of randomly rough surface is applied to model the target with non-uniform unstable roughness surface. Here, the root mean square (RMS) height and the correlation length of rough surface are applied to describe the roughness of different target surfaces. Firstly, the Gaussian surface is generated to model a non-uniform random unstable target, then a combined method of physic optics and equivalent current is employed to simulate the EM scattering in THz band. The scattering characteristics of the target with different roughness scales at different incident angles and radar frequencies are analyzed and discussed.