Diffusion mechanism of rare earth Dy, glow discharge and hollow cathode effect, surface vacancy distribution of W-Mo alloying layer and W-Mo-Dy alloying layer formed on surface of carbon steel by using glow plasma surface alloying technique were studied, and the activation energy of diffusion was calculated by Ficks Second Law.It was shown that under the conditions of ion bombardment and non-balance, the alloy elements put effect on concentration gradient, vacancy concentration and activation energy of diffusion of each other, made diffusion velocity of Dy increased and promoted the diffusion of W, Mo.Through analysis that the rare earth has two possible diffusion model: the Vacancy Exchange Diffusion during preliminary stage, and Rare earthVacancy Complexes Diffusion once the diffusion process is progressed far enough.