Terahertz back scattering intensity is a very important index to represent the scattering ability of targets to terahertz. Firstly, the feasibility of 2.52 THz back scattering properties measurement system is proved by the measurement of the calibration target, and the signal-to-noise ratio of the system is measured. The relative back scattering intensities of rectangular milling and sand blasting aluminum plate with six kinds of sizes are measured in the measurement range of 0°-3°. The experiment results show that the measured maximum dynamic range is 32.25 dB, back scattering intensity difference between sand blasting aluminum plate with different sizes is more obvious than that between milling aluminum plate, the average errors of milling and sand blasting aluminum plates with same surface area at 0° are 1.35 dB and 0.51 dB, respectively.