The principle of a long-period fiber grating (LPFG) rebar corrosion sensor is discussed in detail firstly based on a sensitive characteristic that the resonance spectrum of LPFG changes with refractive index in external environment. The theory combining the refractive index around rebar in concrete and the resonance spectrum is proposed. Then, LPFG spectrum measurement technology is selected to obtain LPFG spectrum curves corresponding with different kinds of refractive index in concrete, and the relationship between the resonance peak wavelength and the state of rebar corrosion in concrete is obtained. The experimental results numerically show that the resonance peak wavelengths increase and then decrease with the increasing corrosion rates. This method which can directly monitor the state of rebar corrosion in concrete is simple and easy to operate. The measurement and transmission sections of the system are completely composed of optical fiber, which can avoid the electronic interference. There is no necessity to use chemical reagent to sign the solutions which are going to be degraded. In conclusion, the late-model LPFG rebar corrosion monitoring system can achieve a real time, rapid, accurate and long distance measurement.