To achieve precise positioning of steel bars inside concrete structures and accurate detection of early corrosion status， this article introduces a terahertz reflectance spectroscopy technique and systematically explores its potential application in non-destructive testing for structural corrosion assessment. The research results indicate that this technology can achieve precise positioning of steel bars inside the structure and accurate measurement of the thickness of the protective layer， with a positioning accuracy within 1.2 mm of the steel bars. Meanwhile， the minimum measurable thickness of the corrosion layer reaches 45 μm， which is sufficient for effective detection of early corrosion of steel bars. Terahertz technology can achieve quantitative visualization imaging of the thickness of the protective layer and corrosion layer within the scanning area， eliminating the influence of uneven distribution of protective layer thickness and providing an innovative and efficient method for early corrosion characterization of reinforced concrete structures.