Combining　the　processing　maps　with　FEA,　the　critical　values　of　reduction　of　diameter　(ΔR)　for　the　appearance　of　as-cast　microstructure,　dynamic　recrystallization　grains　and　twins　in　the　swaged　magnesium　were　respectively　studied,　by　which　the　rational　range　of　ΔR　was　obtained.　To　ensure　that　the　as-cast　microstructure　wouldn＇t　be　retained　in　the　rotary-swaged　magnesium,　ΔR　should　be　larger　than　0.25　mm.　To　avoid　the　appearance　of　twins　in　the　microstructure　of　rotary-swaged　magnesium,　ΔR　should　be　less　than　1.0　mm.　ΔR　should　be　between　0.25　mm　and　1.0　mm　to　ensure　the　appearance　of　recrystallized　grains.　The　surface　roughness　of　rotary-swaged　magnesium　increases　with　the　increase　of　ΔR　by　quadratic　function.　The　simulated　results　accord　well　with　the　experimental　data.　In　the　process　control　of　rotary-swaging(RS),　ΔR　should　be　maximized　between　0.25　mm　and　1.0　mm　on　the　precondition　that　the　requests　for　surface　roughness　have　been　satisfied.