Distributed optical fibers can effectively overcome an insufficient number of measuring points in structural health monitoring and provide additional measuring point information. However, under quasistatic working conditions, effective solutions for identifying damage in bridge structures with respect to distributed optical fibers are scarce. To address this problem, a structural damage diagnosis method for prefabricated beam bridges, which combines a quasistatic strain influence line with distributed optical fiber sensing technology, is proposed. First, quasistatic strain influence line data for multiple sections of the main beam structure of a bridge are obtained via differential pulse pair–Brillouin optical time-domain analysis technology. By calculating the correlation matrix of the strain response data of different tests, a damage characteristic matrix is constructed, a null space matrix of the damage characteristic matrix is obtained, a damage diagnosis factor is constructed, and a damage judgment threshold is established. Second, by comparing the damage index with the diagnostic threshold under healthy conditions, the location of multisection structural damage can be determined. Considering that structural damage will cause changes in stiffness, the concept of a “strain response area” is introduced, and a quantitative indicator of structural damage is constructed. Last, the proposed method is verified by constructing a scaled-down test model for the multisection damage diagnosis of bridges.