With　a　view　to　improving　the　surface　quality　of　the　magnesium　(Mg)　alloy　at　a　warm　temperature,　the　ultrasonic-assisted　warm　single-point　incremental　forming　(SPIF)　is　proposed.　The　surface　quality　of　the　Mg　alloy　during　the　warm　SPIF　is　primarily　affected　by　two　parts:　the　orange　peel　patterns　of　the　non-contact　surface　and　the　scratches　and　adhesive　wear　of　the　contact　surface.　In　this　work,　the　surface　quality　of　the　AZ31B　Mg　alloy　sheet　parts　at　different　forming　temperatures　and　ultrasonic　amplitudes　is　evaluated　by　examining　the　surface　roughness　and　topography.　The　results　show　that　the　generation　of　orange　peel　patterns　is　significantly　affected　by　temperature.　In　addition,　scratches　and　adhesive　wear　of　the　contact　surface　increase　with　the　rising　temperature.　After　applying　the　ultrasonic　vibration　(UV),　the　quality　of　both　the　non-contact　and　the　contact　surfaces　of　the　parts　is　significantly　improved,　but　too　large　ultrasonic　amplitude　slightly　reduces　the　surface　quality.　Moreover,　the　microstructural　examination　results　show　that　UV　has　a　great　effect　on　dynamic　recrystallization　and　grain　refinement,　which　positively　affects　surface　quality　improvement.