In order to clarify the damage identification method applicable to each member type of beam string structures with improved configuration of lower chord and strut, and to reduce the misjudgment at non-damaged members, in this paper, a damage identification index of sub-component curvature variation based on structural mode is proposed, the validity of the index is illustrated by analyzing a simply supported beam structure with a simple structural form. The finite element software ANSYS was used to establish the analysis model of beam string structures with improved configuration of lower chord and strut, the damage of the member is simulated by reducing the elastic modulus of the member element. Considering the symmetry and complexity of the structure, damage conditions of single damage, multiple damage, and small damage level were set up in the mid-span, quarter-span, and near-bearing end bars, respectively, the improved curvature absolute difference index of sub-component and the sub-component curvature variation based on structural mode proposed in this paper are used for the comparative analysis of damage identification. The results show that the index of sub-component curvature variation based on structural mode can achieve the precise localization of single-damaged and multiple-damaged structures, and this index is also very sensitive to working conditions with a small degree of damage, which can be used as an effective method for the damage identification of the beam string structures with improved configuration of lower chord and strut.