Abstract: This study addresses the issue of tool wear in ultrasonic-assisted milling of titanium alloy (Ti-6Al-4V). It systematically analyzes the motion characteristics of a longitudinal-torsional ultrasonic vibration system to understand the trajectory of the tool cutting edge. Using Matlab software, the tool milling trajectory was simulated based on the longitudinal-torsional ultrasonic motion equations combined with measured ultrasonic amplitudes. The trajectory characteristics were analyzed to investigate the influence mechanism of ultrasonic amplitude on tool wear.Through longitudinal-torsional composite ultrasonic milling experiments and wear morphology analysis, it was found that ultrasonic vibration significantly regulates wear behavior but exhibits a critical amplitude threshold. The optimal suppression of tool wear occurred when the torsional amplitude reached 2.252 μm and the longitudinal amplitude reached 1.803 μm, resulting in a flank wear (VB value) of 37.829 μm—a 41.340% reduction compared to conventional milling.Analysis of wear characteristics at the tool tip and cutting edge revealed that longitudinal-torsional ultrasonic vibration effectively reduces chip adhesion. However, excessively high ultrasonic amplitudes cause unstable motion coupling, inducing stress concentration at the tool tip and leading to chipping and coating delamination, which conversely increases wear. By rationally selecting ultrasonic amplitudes, tool wear can be effectively suppressed. Under appropriate amplitudes, ultrasonic vibration reduces heat accumulation during cutting and inhibits adhesive wear, thereby extending tool life and improving machining efficiency. This study provides key parameter guidelines for the application of ultrasonic-assisted milling in Ti-6Al-4V titanium alloy processing, offering significant engineering value for reducing manufacturing costs of difficult-to-machine materials.