A detecting method of investigating the function of crack width versus fiber Bragg grating (FBG) spectrum deformation was proposed. In this paper, the bond-slip model of deformed steel bar in concrete slab was used to calculate the crack width. The stress distributions along optic-fiber grating were extracted by the three-dimensional (3D) finite element method (FEM) and the spectrum deformation of FBG induced by the uneven distribution of the stress was calculated. Experiments were carried out with an FBG fixed on the bottom of concrete slab. The crack width and FBG spectrum deformation of experimental results were given, and the function of crack width versus wavelength shift of FBG sensor was given also. The proposed technology in this paper could improve the safety of large-scale concrete building by early identification of structural crack.