Target surface scattering characteristics are the physical basis of terahertz waves used in radar imaging and target recognition and location. The main factors affecting target surface scattering characteristics are the target material and surface roughness. This study uses metal aluminum as an example and fits the Drude model parameters of aluminum at 0.1 THz. Based on the fitting results, the scattering coefficient of a Gaussian random rough aluminum surface is analyzed using Kirchhoff approximation (KA) method. Subsequently, Monte-Carlo rough targets with different roughness are modeled, and the radar scattering cross-section (RCS) is calculated to image the two-dimensional inverse synthetic aperture radar (ISAR). The research results show that the target RCS simulation results are consistent with KA theory analysis, that is, surface roughness is negatively correlated with RCS at a small pitch angle, whereas surface roughness is positively correlated with RCS at a large pitch angle. In addition, in a certain rough range, as the surface roughness increases, the scattering center of the target surface increases, and the ISAR image forms a dense "speckle" effect, which can better reflect the shape and structure of a target.