Due to the differences in materials, processes and environment, the welded structures are prone to various types of defects, which will seriously affect the overall safety performance of the structure. The ultrasonic method is an effective non-destructive measurement method for defect, but the conventional ultrasonic defect measurement technology has the problem of mode conversion when dealing with the complex curved surfaces, which affects the accuracy of defect detection. Therefore, a simulation study on the unequal thickness butt weld defect detection based on zero-group-velocity Lamb wave mode is carried out in this work. First, based on the finite element characteristic frequency method with Bloch-Floquet boundary and domain constraints, the frequency deviation of the zero-group-velocity mode in a thin plate with variable thickness is studied, and the zero-group-velocity mode sensitive to thickness changes is selected and the parameters of excitation signal are optimized. Then, a welding simulation model is established to verify the frequency deviation law of the zero-group-velocity mode in the thin plate with variable thickness, and the curve thickness of the weld thin plate iss successfully reconstructed with an error of ±0.05 mm. Finally, a welding model of the unequal thickness plate is established for defect detection, and the position information (>0.5 mm) of the small defects inside the weld is successfully reconstructed, and the errors of the thickness direction and the horizontal direction are ±0.05 mm and ±0.02 mm respectively.