To investigate the stress behavior and load-transfer mechanism at the steel-concrete joint segment of a flat arch bridge, this study takes a flat arch bridge in Guangdong with a calculated span of 196 m as the engineering background. Based on the Abaqus platform, a refined finite element model of the steel-concrete composite arch abutment was established to analyze the stress level and distribution characteristics of the structure under six unfavorable load cases. In addition, a 1∶8 scaled patial model was fabricated for multi-condition loading tests to study the mechanical behavior of the steel-concrete joint segment under different scenarios. The results indicate that, under all loading cases, the stress in the bottom plate of the steel arch box is most significant, making it the primary load-bearing component, with a maximum stress of -116.875 MPa. In the stress distribution of the bottom plate, the stiffening ribs play a major role, exhibiting the most prominent stress levels compared to other areas. The web area adjacent to the bottom plate of the steel arch box experience the next highest stresses, with a maximum value of -32.16 MPa across all conditions. The stress level in the top plate of the steel arch box is relatively low. The measured stress values in the concrete pile cap are generally small, with a maximum stress not exceeding -0.73 MPa. The axial compressive stress in the steel arch box gradually decreases from the loading end to the concrete pile cap end. When subjected to loads, the steel arch box primarily transfers the applied loads to the concrete pile cap through the bottom plate and the adjacent web regions. Under all test conditions, the stress level at the steel-concrete joint segment is relatively low, as most of the stress from the bottom plate of the steel arch box is dispersed into the concrete pile cap via components such as PBL shear connectors, bearing plates, densely arranged stiffening ribs, and penetrating reinforcement bars.