Based on the expand behavior of the corrosion product and the strain and temperature measurement principle of fiber Bragg grating (FBG), a sensitive FBG reinforcement corrosion sensor is proposed. The sensor consists of two FBG sensors and a customized rebar which has an axial through-hole, an axial groove, a ring groove, and a blind hole. The FBG-1 sensor is designed to monitor the expansion strain caused by steel bar corrosion, and the FBG-2 sensor is used as a temperature compensator to eliminate the cross-sensitivity of the FBG-1 sensor. In order to increase the survival rate of the FBG corrosion sensor, a layer of filter paper is tightly wrapped outside the FBG-1 sensor and the FBG corrosion sensor is sealed with cement mortar. The calculation formula of reinforcement corrosion rate can be calculated according to the wavelength value collected by FBG interrogation system. In addition, based on Faraday’s law of electrolysis, electrochemical accelerated corrosion experiment is designed to explore the working performance of the FBG corrosion sensor. The experimental results show that the sensor can accurately monitor mass loss rate within 0.7%, the measurement range of which is greater than 5%.The sensor can effectively monitor the early corrosion process of reinforcement, which will minimize maintenance costs.