The deflection change of concrete beams will cause concrete cracking and reinforcement corrosion, which will affect the use function of the concrete beam structures such as bridges and tunnels etc. Distributed optical fiber sensing (DOFS) was introduced to measure the deflection distribution of the concrete bridge structures in case avoiding the weakness of traditional monitoring methods in this paper. Above all, based on the analysis of the principle of DOFS Brillouin Optical Frequency Domain Analysis (BOFDA) measurement, the layout and deployment of optical fibres on the bottom of the concrete beam in a bridge for deflection measurement is proposed. Then, according to the fiber optic strain distribution, the qualitative characterization of the concrete bridges’ deflection was studied. Later, the mathematical expression of the optical fiber strain quantifying the bridge deflection was deducted theoretically through the analysis of the geometric principle of the optical fiber deformation when the concrete bridge deflection occurred. Finally, the indoor model experiment simulating the concrete bridge deflection has been carried out based on BOFDA. The results show that the concrete bridges’ deflection measured by BOFDA was the same as that measured by the dial indicators. BOFDA cannot only characterize the degree of concrete bridges’ deflection, but also accurately denote the deformation range and deflection distribution. It shows that BOFDA technology is effective and feasible for deflection measurement of concrete beam bridges.