Abstract: Mode II debonding is one of the most typical failure modes on carbon fiber reinforce polymer (CFRP) plate-to-steel interface. In order to elucidate the interfacial debonding mechanism of CFRP-strengthened steel structures, single- and double-lap shear tests were conducted to investigate the mechanical behavior and debonding failure process at the FRP-to-steel plate interface. The technique of digital image correlation (DIC) was applied to measure the normal strain distribution on the surface of CFRP. Comparing the failure modes between these two joints, it can be found that a mode II fracture occurs on the interface of double-lap joint, which is caused by shear stresses. While the interface of single-lap joint subjected to eccentric loading is governed by mixed-mode I/II behavior, with the failure attributed to the combination of shear stress and bending moment. Additionally, the ultimate load and bond slip of the double-lap joints in mode II increase with the increment of bond length until an effective bond length is reached, beyond which the ultimate load remains unchanged. However, for single-lap joints subjected to eccentric loading, the ultimate loads with different bond lengths are almost the same. Based on bilinear bond-slip law obtained from experiment results, finite element model can be established to analysis the mode II interfacial debonding process of CFRP-strengthened steel structures, which shows an excellent agreement with the experimental results.