<p>Aiming at enhancing the fatigue performance of the laser-welded skin-stringer joints used in aircraft fuselage panels, this study investigates the effect of high frequency mechanical impact treatment (HFMI) on the fatigue performance of 3-mm-thick 7075-T6 aluminum alloy thin-walled T-welded joints. Four-point bending fatigue tests were conducted on as-welded and HFMI-treated specimens. The fatigue fracture surfaces, crack propagation path, and the treated weld toe of the tested specimens were examined by electron microscope and optical microscope. Fatigue crack propagation analysis was conducted by using extended finite element method (XFEM) to evaluate the HFMI-induced effects on fatigue. The fatigue test results indicate that HFMI can also effectively increase the fatigue life of thin-walled welded joints, and the improvement is higher in the region of low stress range. Increasing the excitation amplitude of the ultrasonic transducer reduces the fatigue improvement. Due to the special features of thin-walled welded joints, tensile transverse stresses and widely distributed fold defects are introduced in the surface layer of 100 ~ 200 μm by HFMI. Determined by the orientation of the defects with the loading direction, the transverse defects act as fatigue crack sources, while the longitudinal defects have less effect on fatigue damage. Fatigue crack propagation simulation indicates that the benefit of compressive residual stress beneath the surface on delaying fatigue crack growth exceeds the negative effects of surface tensile stress and fold defects on fatigue crack initiation, which contributes to significant fatigue enhancement.</p>