Terahertz back scattering properties is a very important index to represent the target′s scattering ability of terahertz. Paraboloid occupies an important position in practical applications, but the correlational studies on 2.52 THz target back scattering properties still remain untouched so far. The back scattering measurement experiments on paraboloid with diameter of 50, 60, 70 and 80 mm based on measurement system were conducted. The roughness of paraboloid was 3.2, 6.3, 12.5 μm and so on. The MATLAB software was used to deal with the data and the target back scattering properties curve was got. The controlling variable method was used to study size and roughness effects of target terahertz back scattering properties. Target back scattering properties curve dropped slowly as it grew in size, and the lug boss nearby 3° was becoming more and more obvious. The signal waveform of measuring angle edge was observed by the oscillograph which proved the signal was available. Back scattering experiment results show that the maximum measurement dynamic range is 22 dB and the maximum measurement error is about 0.15 dB.