The consideration of non-ideal supporting boundary and uncertainty of section parameters in actual bridge structure are insufficient in the research of beam bridge structure damage identification. Thus a beam structural model considering elastic rotation and vertical restraint was proposed, and the uncertainty coefficient of section parameters and local damage were introduced to simulate the existing beam structure. The analytical expression of the rotational-angle-influence-lines of elastic-constrained-support beam and the damage identification indicator based on that were derived, and the loading implementation scheme was proposed. The influence of measuring point, damage degree and measuring noise on the damage identification was investigated combined with the simulation cases. The experiment of the boundary equivalent substructure model was carried out to verify the proposed method. Research show that the curvature of rotational-angle-influence-lines difference can be used to accurately locate the local damage and effectively calculate the degree of existing beam structural damage, and the maximum relative errors of the damage degree solution are 25%. Simulation cases show that the proposed method can locate the damage positions under the condition of 5% test noise. The susceptibility of damage diagnosis is high when the rotation constraint is weak.