Abstract: Characteristics of cross girder distortion and the Bragg wavelength shift of fiber Bragg grating (FBG) are analyzed. A method for measuring minute vertical displacements in cross girders is proposed. The principles in sensing is deduced theoretically. The shift of central Bragg wavelength is measured under different conditions of distortion displacement. The experiment accords well with theoretical deductions in this analysis. The precision of measurement is in direct proportion to the effective index and spatial period of the FBG, and also the thickness of the girder,but in inverse proportion to the square of span of the girder. The scope of measurement has relationships in the contrary direction.