Abstract: Fiber reinforced polymer (FRP) has been effectively used in strengthening steel structures, but FRP-steel interfacial debonding may trigger structural failure. Hence nondestructive testing is required to ensure structural safety. Herein one specimen with FRP-steel interfacial debonding was detected based on a self-developed eddy current thermography (ECT) system, and its heat generation and heat transferring mechanism was revealed through multi-physical numerical simulation. The effect of different shapes, locations and shapes of the defect on the temperature response under ECT was investigated through simulation. The results show that the ECT system can accurately detect 328 mm² in the specimen within 80 s. The edge effect affects the detection accuracy of the edges and corners of the specimen in the heating stage, but it has little effect due to heat redistribution in the sample in the cooling stage. The blur effect caused by heat conduction along the depth direction reduces the detection accuracy of deep defects. Numerical simulation shows good agreement with the experimental tempera-ture response, and reproduces the edge effect and blur effect of ECT. Parametric study using numerical simulation reveals that the heating rate can overcome the edge effect and blur effect in a certain range and improve the accuracy of FRP-steel interfacial defect detection.