To solve the problem that the existing nondestructive testing (NDT) technology is difficult to accurately detect the early corrosion thickness of steel plate, based on the non-contact nondestructive terahertz time-domain spectroscopy (THz-TDS) technology, the optical parameters of corrosion products and the corrosion thickness of steel plate are detected by using the high transmission to non-polar materials and reflection to polar steel materials properties of terahertz (THz) waves. The experimental results show that when the effective frequency is within the range of 0.2-1.6 THz, the refractive index of the corrosion products is approximately 2.8. The relative amplitude of the first peak in the reflective THz signal has a logarithmic function relationship with the corrosion time of the steel plate and a linear relationship with the reciprocal of the corrosion time. The experimental results also show that the corrosion thickness increases linearly with the increase of corrosion time, and the accuracy of corrosion thickness measured by THz technology is more than 90%. The feasibility and accuracy of non-contact THz-TDS technology for corrosion thickness detection are therefore proved.