The scattering characteristics and the affecting factors of rough metal surface were analyzed based on the Kirchhoff approximation theory. Rough aluminum (Al) plates with different roughness were manufactured and their scattering characteristics were measured by two systems: a single-frequency system built with a far-infrared (IR) laser and a wide-band Fourier Transform IR spectroscopy (FTIR) system. Good consistency was found from the comparison of theoretical and experimental results. It is proved that the peak scattering coefficient has negative correlation with roughness and frequency, and positive correlation with incident angle. Additionally, the applicability of Kirchhoff approximation theory in the terahertz range was given by analyzing two limiting cases for near-smooth and high-roughness samples. The conclusions lie the foundation for theoretical calculation of complicated targets and will promote the development of the theories and techniques of terahertz radar.