To address sectional damage occurring in either the steel girder or the concrete slab, this paper derived the Deflection Influence Line (DIL) for an arbitrary cross-section. Damage identification was performed based on the difference of DIL (DDIL) before and after damage and its second-order difference, and the method was then validated through finite element analysis and model tests. Finally, a damage identification method using the symmetric DDIL and its second-order difference under service conditions was proposed. The results show that damage localization can be achieved by identifying the inflection point of the DDIL curve or the peak on its second-order difference curve. Both the DDIL and its second-order difference increase approximately linearly with increasing damage severity and damage length. Model tests indicate that the DDIL yields better damage identification performance than its second-order difference. The proposed method based solely on the symmetric DDIL and its second-order difference under service conditions does not require pre-damage deflection data.