The　plasticity　of　magnesium　alloys　is　inherently　constrained　by　their　hexagonal　close-packed　(HCP)　crystal　structure　and　limited　slip　system　at　room-temperature,　which　restricts　their　widespread　application　across　various　industries.　Therefore,　identifying　an　effective　method　to　enhance　the　plasticity　and　formability　of　magnesium　alloys　remains　essential.　In　this　study,　the　mechanical　behavior　of　AZ31B　Mg　alloy　was　examined　under　the　combined　influence　of　ultrasonic　vibration　(UV)　and　a　thermal　field.　Tensile　tests　incorporating　UV　and　thermal　assistance　were　performed　on　the　sheets　at　amplitudes　ranging　from　0　to　50.7　µm　and　strain　rates　between　10–2　and　10–4　s–1　at　a　temperature　of　150　°C.　Microstructural　evolution　during　deformation　was　analyzed　using　optical　microscopy　(OM)　and　electron　backscattered　diffraction　(EBSD).　The　results　indicate　that　under　hybrid　energy　fields,　the　interaction　between　UV　and　strain　rate　significantly　affects　the　flow　stress,　elongation,　and　the　critical　strain　required　for　dynamic　recrystallization　(DRX)　in　Mg　alloys.　Furthermore,　microstructural　analysis　reveals　that　the　incorporation　of　UV　within　the　thermal　field　facilitates　intra-grain　rotation　and　deformation,　promotes　DRX,　particularly　continuous　DRX,　and　enables　dislocation　migration　from　the　grain　boundary　to　the　grain　interior.　Consequently,　a　notable　improvement　in　plasticity　is　observed　across　the　tested　strain　rate　range　when　UV　is　applied　at　suitable　amplitudes.　However,　excessive　amplitudes　lead　to　contrasting　variations　in　mechanical　behavior,　DRX　extent,　and　dislocation　movement.　Additionally,　the　underlying　mechanisms　responsible　for　these　effects　have　been　clarified.　©　2025　Elsevier　Ltd