Abstract: As one of the most common connection methods in storage tanks and pipelines, butt welds typically require inspection methods beyond standard magnetic flux leakage (MFL) which primarily targets the base material to improve detection efficiency. In this study, a butt weld MFL detection model was constructed based on MFL principles. Using finite element analysis, simulations were performed by varying the geometric parameters of three types of welding defects, including root incomplete penetration, sidewall lack of fusion, and undercut to investigate the superimposed MFL signals at both the weld and defect locations. The results showed that as the depth of the welding defects increased, the inner peak-to-valley values of the radial component of the magnetic flux density and the peak values of the axial component gradually rised, making the signal superposition at the weld more pronounced. Under laboratory conditions, these three types of welding defects were fabricated, and experiments were conducted to verify how different defect geometrical characteristics influence MFL signals. This work provided theoretical insights for the on-site application of MFL technology in butt weld inspection.