The work aims to obtain the metal diffusion coating by the surface solid diffusion technology and study the formation mechanism of the diffusion coating. Solid diffusion Al and Zn was prepared on AZ81 magnesium alloy surface at 390 ℃, the phase constituents and morphologies of the diffusion alloyed coating were analyzed by X-ray and optical microscope. And the diffusion formation mechanism was studied by building diffusion processing model. When the diffusion time changed from 4 h to 8 h, the alloy coating changed as follows except the original phases ?-Mg+Mg17Al12 in AZ81 magnesium alloy: Mg0.97Zn0.03→Mg7Zn3+Mg2Zn3+Mg2Zn11+AlMg2Zn→Mg32(Al,Zn)49+AlMg4Zn11+AlMg2Zn. The mechanism of surface diffusion of Al and Zn on AZ81 magnesium alloy was vacancy diffusion at the initial stage. Al and Zn were dissolved in AZ81 magnesium alloy matrix surface, Zn firstly reached to the matrix solid solubility and then Mg and Zn reacted to generate compounds. However, the Mg and Zn compounds were thermodynamically unstable and decomposition occurred at the diffusion temperature of 390 ℃ and the more stable Mg-Al-Zn compounds were formed. After the presence of Mg-Zn compounds, the formation mechanism of the diffusion coating was combined with vacancy diffusion, reaction diffusion and melt decomposition and the diffusion and reaction diffusion process were accelerated by melt decomposition of Mg-Zn compounds. The thickness, phase constituents, phase size and morphology of diffusion coating for solid diffusion Al and Zn on AZ81 magnesium alloy change gradually with prolonged diffusion holding time at the same temperature. The main formation mechanism of the diffusion coating changes to reaction diffusion from physical diffusion.