A homodyne detection system to acquire the thickness of silicon wafers is constructed and described. By harnessing the relationship between the transmission phase change of a 4.3-THz light beam and the incident angle controlled by a mechanical rotating stage， the thickness value of sample can be precisely deduced using the standard residual error method. The results indicate that the fitted thickness of the sample differs by only 2.5~3 μm from more accurate results measured by optical microscopes， achieving terahertz non-destructive thickness measurement with micron level accuracy. The experiment validates the effectiveness of terahertz quantum-cascade laser in non-contact and nondestructive measurement.