In-service reinforced concrete bridges, following prolonged operational periods, frequently undergo alterations in structural integrity and significant deterioration of material properties. As a result, it becomes imperative to evaluate and analyze these bridge structures. This study focuses on a specific in-service concrete bridge and employs finite element simulation analysis to investigate the failure behavior and degradation of load-bearing capacity in concrete beam bridge structures. The research examines the impact of various crack distributions and types, specifically addressing the effects of bending and shear cracks on the failure behavior and degradation of load-bearing capacity in concrete bridge structures. Load simulations are conducted on the bridge, revealing that bending cracks exert a relatively minor influence on the failure mode of the structures. The load-deflection curves demonstrate minimal variation across different crack heights, indicating that the structures do not experience abrupt brittle failure, and their structural performance is largely optimized in this context. Conversely, shear cracks have a pronounced effect on the failure mode of the bridge structures. Notably, when the crack height reaches 0.6h, the load-deflection curve exhibits a significant alteration, leading to brittle failure attributed to shear cracks, thereby indicating that the structural performance is not fully realized. Given that the deformations in cracked bridge structures comprise two components, stiffness reduction formulas are introduced. Utilizing the stiffness reduction formula outlined in the standard (JCT3362—2018) for cracked components, a formula for calculating residual load-bearing capacity is derived. Calculations pertaining to the mid-span section load-bearing capacity of the selected bridge reveal deviations within 5%. This study offers a valuable methodology and reference for assessing the residual load-bearing capacity of reinforced concrete bridges exhibiting crack damage.