Due to the unevenness of the corrosion layer and effect of the coatings, it is difficult to accurately detect the corrosion thickness of steel plates by the existing nondestructive testing (NDT) approaches. Based on the properties of the high transmission to non-polar materials and reflection on the surface of polar metal materials of terahertz (THz) wave, a nondestructive terahertz time-domain spectroscopy (THz-TDS) is utilized to detect the corrosion thickness under coatings in this paper. The experimental results show that in the effective frequency range of 0.2-1.2 THz, the refractive index of corrosion products, epoxy resin, rubber and cement paste is 2.80, 1.94, 2.18 and 2.04, respectively. The delay time difference between the sample and the reference signals in transmission THz-TDS exhibits a linear relationship with the refractive index. The reflected THz signal can identify the corrosion layer on steel plate surface, and accurately measure the corrosion layer with thickness greater than 40 μm. The corrosion of steel plates under coatings can be judged by the number and attenuation of signal amplitude in reflection THz-TDS. The thicknesses of the coatings and corrosion layer are determined with accuracy greater than 90% by the delay time difference corresponding to the signal amplitudes. The application and accuracy of THz-TDS for NDT corrosion thickness detection of steel plates under coatings are therefore proved.