The external confinement of steel pipes with slot-type defects using carbon fiber-reinforced polymer (CFRP) has emerged as an effective repair approach. However, interface debonding between CFRP and the steel substrate often governs the failure of such strengthened systems, and its fracture mechanism has not been fully clarified. This study combines experimental testing with theoretical modeling to investigate the debonding process. A specially designed bonded joint was employed in the experiments, while distributed optical fiber sensors (DOFS) were used to capture the strain evolution of CFRP during loading. The experimental findings reveal that debonding proceeds through three distinct stages: an initial elastic response, a rapid crack propagation (RCP) stage, and a steady-state crack propagation (SSCP) stage. To interpret these observations, analytical models were developed. A cohesive zone framework with a linear elastic-brittle traction-separation law was adopted to describe the interface behavior in the elastic and RCP stages, where mode I fracture dominates. The subsequent SSCP stage was interpreted as a mixed mode I-II fracture process using Griffith’s fracture criterion. Good agreement between model predictions and test results demonstrates the reliability of the proposed approach.