Crack detection is very important for long-term large-scale structural health monitoring(SHM).Fiber optic sensor has been approved to have good performance in application in SHM.In this paper,we present a novel method to monitoring the crack by using long-gauge FBG sensor.A FBG with a gauge length of 50 mm is fabricated.When the crack pass through the FBG,the reflect spectrum of FBG will be expanded or chipped.Different position of the crack will result in different Shape of the spectrum.By analyzing the characteristics of the spectrum,the position of the crack can be detected and the opening width of the crack can be estimated.