In　order　to　discuss　the　problem　of　definite　quantity　of　the　carbon　migration　in　plasma　surface　alloying　process,　the　diffusion　of　alloy　and　carbon　element　was　analyzed.　The　solutions　to　the　difference　equation　were　supplied　on　the　depth　of　the　alloying　layer,　the　decarburized　layer,　the　distribution　of　alloy　elements　and　carbon　in　the　layer.　The　numerical　calculation　and　the　experimentation　were　accomplished　with　certain　technical　parameters.　The　results　show　that　the　numerical　value　of　calculation　is　consistent　with　the　experimentation　respectively　on　the　depth　of　the　alloying　layer,　the　decarburized　layer,　the　distributing　of　alloy　elements　and　carbon　in　the　layer.　The　error　of　calculation　is　less　than　5%　on　the　depth　of　the　alloying　layer　and　is　less　than　10%　on　the　depth　of　the　overall　decarburized　layer.