Terahertz target scattering characteristic is a very important index to represent the target′s scattering ability of Terahertz. A system to measure target′s scattering characteristic of 2.52 THz is established. The system has a shield compartment, and the beam′s waist width is about 100 mm. With this system, a back scattering measurement experiment is conducted on square plates whose roughness Ra are respectively 1.6, 3.2, 6.3, 12.5 μm. The square plates are 40 mm and 45 mm wide. Through analyzing the experimental data and comparing them with the theoretical formula, the feasibility of this system is verified. The experimental results show that the back scattering′s maximum value of the aluminum plates with different roughnesses changes little, when the size of the aluminum plates is the same. While the roughness increases, the back scattering declines from the peak and the gradient trends greater.