Abstract: To explore the bond performance and damage progression at the interface between carbon fiber-reinforced polymer (CFRP) and steel under constant amplitude fatigue loading, fourteen CFRP-to-steel single-lap shear joints, fabricated using the nonlinear structural adhesive Araldite 2015, were tested under both monotonic and fatigue loadings. The influence of load ratio on failure mode, fatigue life, and bond performance was examined, shedding light on the damage evolution at the CFRP-steel interface under fatigue. Under monotonic loading and constant amplitude fatigue loading, CFRP-steel single-lap shear specimens all experienced failure within the adhesive layer. As the load ratio decreased, the residual adhesive on the steel surface decreased. The fatigue life of the interfaces decreases with the increased load ratio, with a power-law relationship observed between fatigue life and load ratio. During fatigue loading, the stiffness at the loading end of the specimen progressively degraded with fatigue cycles, particularly noticeable at higher load ratios. Analysis of CFRP strain distribution and interfacial shear stress suggested that the cohesive failure under fatigue loading initiated at the loading end and propagated towards the free end as a result of damage accumulation. An increase in the load ratio resulted in higher peak strain at the CFRP laminate at failure, although the value remained lower than that under monotonic loading. The peak shear stress first increased and then decreased with the increasing of load ratio. At high load ratios (≥0.7), the peak shear stress exceeded that under monotonic loading. The bond-slip response evolved from approximately bilinear at the load ratio of 0.3 to trilinear at the load ratio of 0.8, with the latter showing a more extended plateau than that at the load ratio of 0.7. Stiffness degradation under a single cycle of fatigue loading indicated varying damage accumulation rates. Within 67.5 mm from the loading end, the rate was lower compared with regions beyond 67.5 mm, where the rates were approximately the same. This study analyzed the influence of load ratios on the bond performance of CFRP-steel interface under fatigue loading, providing valuable insights for research on the fatigue behavior of CFRP-strengthened steel structures.