Abstract: Ti6Al4V alloy is critical for thin-walled aerospace components, yet conventional methods for its surface enhancement struggle to balance efficiency and precision. While ultrasonic vibration milling has been demonstrated to improve fatigue performance, its strengthening mechanism requires further investigation. Additionally, its application in fatigue-critical side milling remains underexplored. To address this gap, we introduce the method of ultrasonic peening side milling (UPSM), which integrates elliptical vibration into side milling to achieve simultaneous machining and surface strength-ening. Theoretical and finite element analyses are performed to elucidate the mechanisms of residual stress generation and plastic deformation in UPSM and two-pass UPSM (TUPSM). Our experimental results demonstrate that the UPSM method reduces surface defects. At a vibration amplitude of 8 μm, UPSM increases the surface compressive residual stress by 47.4% and the subsurface plastic deformation layer thickness by 91.5% as compared to conventional milling (CM). TUPSM amplifies these effects, achieving a 55.5% increase in residual compressive stress. Fatigue tests reveal 3.38-fold (for UPSM) and 3.76-fold (for TUPSM) improvement in fatigue life over CM, a phenomenon which is attributed to the subsurface crack initiation and grain refinement induced by ultrasonic ironing and impact effects. This work estab-lishes UPSM as an integrated, cost-effective solution for enhancing fatigue performance in thin-walled Ti6Al4V compo-nents, overcoming the limitations of conventional methods and advancing between precision machining and strength-ening treatments.