In order to clarify the damage identification method applicable to each member type of beam string structures with an improved configuration of lower chords and struts, and to reduce misjudgments at non-damaged members, this paper proposes a damage identification index based on sub-component curvature variation and structural modes. The validity of the index was 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 an improved configuration of lower chords and struts. The damage of the members was simulated by reducing the elastic modulus of the member elements. Considering the symmetry and complexity of the structure, damage conditions of single damage, multiple damage, and small damage level were set up on the members at the mid-span, quarter-span, and near the bearing ends, respectively. The improved curvature absolute difference index of sub-components and the sub-component curvature variation based on structural mode proposed in this paper were used for the comparative analysis of damage identification. The findings demonstrate that the index of sub-component curvature variation based on structural modes can achieve the precise localization of single and multiple damages in the structure. This index is also highly sensitive to slight damage, making it an effective method for identifying damage to beam string structures with an improved configuration of lower chords and struts.