Corrosion of main steel reinforcement is one of the most significant causes of structuraldeterioration and durability reduction. This research proposes a two-level detection strategy tolocate and quantify corrosion damage via a new kind of long-gauge fiber Bragg grating (FBG) sensor.Compared with the traditional point strain gauges, this new sensor has been developed for bothlocal and global structural monitoring by measuring the averaged strain within a long gauge length.Based on the dynamic macrostrain responses of FBG sensors, the strain flexibility of structures areidentified for corrosion locating (Level 1), and then the corrosion is quantified (Level 2) in terms ofreduction of sectional stiffness of reinforcement through the sensitivity analysis of strain flexibility.The two-level strategy has the merit of reducing the number of unknown structural parametersthrough corrosion damage location (Level 1), which guarantees that the corrosion quantification(Level 2) can be performed efficiently in a reduced domain. Both numerical and experimentalexamples have been studied to reveal the ability of distributed long-gauge FBG sensors for corrosionlocalization and quantification.