Abstract: 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.