Abstract: The bond performance at the CFRP-steel interface determines the reinforcement effectiveness of CFRP materials, with the uneven thickness of the adhesive layer being a critical factor affecting the bonding performance. To reveal the mechanical failure mechanism of CFRP-reinforced steel plates under conditions of uneven adhesive layer thickness, 16 double-shear tests with uniform adhesive layer thicknesses were conducted first. The test results showed that a thickness of 0.5 mm yielded the optimal bearing capacity. Then, 18 additional double-shear tests were performed with the adhesive layer thickness varied both longitudinally and transversely around 0.5 mm to study the mechanical performance and failure mechanisms under uneven adhesive layer thicknesses. The results indicated that the uneven thickness of the adhesive layer significantly impacts the bearing capacity. As the degree of unevenness increased, the bearing capacity showed a decreasing trend. Longitudinal unevenness reduced the bearing capacity by 6.45% to 36.55%, while transverse unevenness reduced it by 9.57% to 47.38%. When the degree of unevenness was the same, the bearing capacity for transverse unevenness was, on average, 9.8 kN lower than that of longitudinal unevenness. The reduction in bearing capacity for transverse unevenness was, on average, 6.65% greater than for longitudinal unevenness, indicating that transverse unevenness has a more detrimental effect than longitudinal unevenness. The strain and shear stress variation patterns of specimens with longitudinal and transverse unevenness also showed significant differences compared to those with uniform thickness. A cohesive numerical model was established based on the bond-slip relationship obtained from the experiments. By analyzing the results of the numerical simulation and the experimental data, it was shown that the cohesive model can effectively simulate the nonlinear mechanical behavior of the bond interface under uneven adhesive layer thickness conditions.