Abstract: 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.