By　using　finite-element　software　program,　MSC/Marc,　and　coupled　thermomechanical　model　the　rotary　swaging　process　of　pure　magnesium　was　simulated.　Distributions　and　changes　of　stress,　strain　and　temperature　are　clarified　and　the　critical　conditions　predicting　two　deformation　defects　are　obtained.　The　deformation　zone　can　be　divided　into　the　bulging　head-end,　the　formed　zone　and　the　reduction　zone.　In　the　formed　zone,　the　equivalent　strain　distributes　inhomogeneously　along　the　radial　direction.　The　equivalent　strain　in　the　center　is　about　half　of　that　at　the　edge.　The　critical　conditions　of　preventing　insufficient　forging　and　edge-cracking　are　(epsilon)　over　bar　(c)　＞=　epsilon(o),　(epsilon)　over　bar　(e)　＜　epsilon(b),　respectively.　The　simulated　height　of　spiral　spine-like　ridge　is　0.0037　mm　and　the　error　is　7.5%.　The　residual　stresses　are　between　18　MPa　and　30　MPa.　Temperature-rising　owing　to　the　heat　translation　from　plastic　deformation　energy　is　not　notable.