The invention discloses a discloses a method for monitoring the corrosion cracks of reinforced concrete based on distributed optical fiber sensing technique. The method comprises the following steps: periodically detecting the polarization current of a steel bar and a stainless steel segment embedded in a sensor by an electrochemical work station to determine the initial corrosion time of the steel bar; monitoring the optical fiber strain by the distributed optical fiber sensing technique in real time through using sensing optical fibers surrounding the steel bar and laid in the sensor to deduce the corrosion expansion or corrosion crack stage of the reinforced concrete; and determining the corrosion expansion or corrosion crack degree by a calibration curve. The invention also discloses a sensor for executing the above method. The sensor can be used for monitoring the total corrosion cracking process of reinforced concrete at any position without damaging the structure of the concrete by monitoring the polarization current and the sensing optical fiber strain of the corroded steel bar, and can determine the corrosion crack stage and the degree of the reinforced concrete. The sensor is used for monitoring and evaluating the durability of reinforced concrete structure in the field of civil engineering.