A new type of corrosion sensor for reinforcing steel was developed in this paper. The optical fiber grating was pre-strained when it was fixed on the surface of cylindrate steel bar. The strain would be released when the steel was eroded,the wavelength of FBG will be changed accordingly. The corrosion depth could be measured by detecting the wavelength of fiber grating. The precision was better than 0.1 μm and the measuring depth scope of 12 μm could be obtained. The sensor could be used to monitor the early corrosion of reinforcing steel in concrete structure.