The microstructure and mechanical properties of 2205 dual-phase pre-strained stainless steel following the high-power ultrasonic treatment (UST) were investigated. Mitigation of the deformation fiber texture, decrease in dislocation density, XRD peak shifting, and hardness reduction corroborated that the ultrasonic treatment can effectively reduce the internal stress of the duplex structure. The EDS and XRD results showed no brittle intermetallic phases were formed after the UST process, unlike conventional thermal treatment methods. Around 18% reduction in microhardness value and a significant ductility improvement of 54% can be achieved using an intermediate amplitude (power) vibration. The microstructure and mechanical properties variation caused by the UST process and the related mechanisms have been discussed in detail. New microstructural mechanisms have been proposed for the interpretation of the observed acoustic stress relief in both α and γ-phases.