The　research　of　plasma　surface　alloying　on　carbon　steels　are　processed　under　different　technique　conditions,　for　approaching　the　principle　of　carbon　migration　in　steel　by　the　double-glow　discharge　plasma　surface　alloying　process.　The　results　indicate　that　when　carbon　atoms　diffuse　in　the　surface,　decarbonized　layer　forms　in　the　frontier　of　the　alloying　layer;　the　strong　decarbonization　phenomenon　in　the　double-glow　discharge　plasma　surface　alloying　technology　is　the　co-result　of　the　surface　sputtering　effect　and　the　diffusion　of　the　carbon　atoms.　The　decarbonized　layer,　which　has　two　kinds　of　microstructure,　namely,　equal-axes　crystal　and　icicle　crystal,　will　be　deeper　with　the　increase　of　cathode　voltage,　the　temperature,　and　the　retaining　time.