<p>To effectively regulate weld defects and improve the microstructural properties of welded joints, the ultrasonic impact treatment (UIT) technique was employed for surface strengthening in this study. The influences of UIT on the state of residual stress, microstructure, nano-hardness, tensile properties, and fracture morphology of the welded joints were systematically analyzed. Furthermore, the residual stress within the micron-scale plastic deformation layer on the welded joint surface before and after UIT was characterized by nanoindentation combined with the Suresh model. The approach was employed to investigate the mechanism of the mechanical property enhancement imparted by UIT to the welded joints. The findings demonstrated that a transformation from tensile residual stress to compressive residual stress within the plastically deformed layer was revealed by nanoindentation characterization. Simultaneously, grain refinement is achieved within the plastically deformed layer, resulting in nano-hardness, bearing capacity, and resistance to plastic deformation being enhanced. Additionally, the ultimate tensile strength of the welded joints was increased by UIT. Based on fracture morphology analysis, uniformly distributed dimples were formed in the fibrous zone, with an increase in the average size of the dimples and the second-phase particles were rendered more homogeneously. The above results indicated that the comprehensive mechanical properties of welded joints were confirmed to be significantly enhanced by UIT, owing to the regulation of residual stress state, refinement of surface grains, and improvement in fracture behavior. An important theoretical basis and technical support were provided by this study for the process optimization and performance improvement of UIT technology in the field of welding manufacturing.</p>