Based on the principle of prestress release and strain measurement of the fiber Bragg grating (FBG), an optical fiber grating sensor for monitoring the corrosion of underwater reinforcing steel was proposed in this paper. As the core component of the sensor, FBG1 was prestretched and tightly wound on the surface of the finely fabricated reinforcing bar for monitoring the prestressing relaxation caused by the loss of corrosion products, and FBG2 was installed in the axial throughhole of the reinforcing bar for eliminating the crosssensitivity of FBG1. In order to improve the survival rate of the sensors, the sensors were embedded into mortar specimens, and the evaluation formula to evaluate mass loss rate of steel rebar was derived.Theperformance of the sensor wasresearched inadesignedelectrochemicalcorrosionaccelerationexperiment, and the corrosion degree could be measured by detecting the wavelength of FBG.By the central wavelength, the corrosion degree of reinforcing steel would be monitored. The experimental results show that the sensor can beusedtomonitor the steelrebar corrosion ofunderwater reinforced concretestructure with large monitoring range, has high sensitivity and good stability. The sensor can be employed to monitor the steel bar corrosion in reinforced concrete structure, such as harbor and pier, bridge pier, dam, reservoir and so on.