The　effects　of　temperature　and　ultrasonic　vibration　on　the　mechanical　behavior　of　AZ31B　magnesium　alloy　sheet　were　studied　by　ultrasonic　vibration　assisted　hot　tensile　tests.　The　results　show　that　with　the　applying　of　the　ultrasonic　vibration,　the　yield　strength　and　tensile　strength　are　decreased,　the　elongation　and　plasticity　are　improved,　and　the　onset　of　the　dynamic　recrystallization　is　delayed.　With　the　increase　of　the　amplitude,　the　yield　strength　and　tensile　strength　further　decrease,　while　the　elongation　first　increases　and　then　decreases.　Comparing　with　that　without　ultrasonic　vibration,　the　elongation　under　the　vibration　of　9.1　µm　increases　by　32.3%　(150　)　and　23.2%　(200　),　respectively,　to　the　maximum　extent　among　the　experimental　results.　Based　on　the　thermal　activation　mechanism　and　dislocation　density　evolution　theory,　the　constitutive　model　used　to　describe　the　hot　tensile　behavior　of　Mg　alloy　sheet　under　ultrasonic　vibration　is　established.　The　performance　of　this　model　is　evaluated　by　the　experiment.　The　model　can　effectively　predict　the　stress-strain　responses　of　materials　under　different　temperatures　and　amplitudes,　and　the　predictive　curves　are　in　good　agreement　with　the　experimental　curves.　It　provides　a　theoretical　basis　for　the　finite　element　simulation　of　metal　sheet　in　hot　plastic　forming　under　ultrasonic　vibration.　©　2022,　China　Science　Publishing　&　Media　Ltd.　All　right　reserved.