Double glow plasma surface metallurgy technology was used on the surfaces of Q235 Steel to form W-Mo-Y alloying layer. The distribution of alloying elements in the diffusion layer was analyzed. The diffusion coefficient and diffusion activation energy of tungsten and molybdenum atoms were calculated to analyze the effect of RE (Y) on the atomic diffusion of W and Mo. The results show that RE (Y) addition increases the diffusion coefficient of molybdenum atoms by 1.89 times in the diffusion layer, but reduces the diffusion coefficient of tungsten atoms by 0.89 time in average. RE (Y) addition decreases the diffusion activation energy of molybdenum atoms by 5737.72, 6511.72 and 7853.38 J/mol in the diffusion layer from the surface of 12~15, 24~25 and 35~36 μm, while increases the diffusion activation energy of tungsten atoms by 998.5, 106.37 and 2904.56 J/ mol. It is indicated that RE (Y) has a catalytic effect on molybdenum atoms but plays no obvious catalytic role in the diffusion of tungsten atoms