The surface modification treatment on pure magnesium was conducted at 390 ℃ by atmosphere solid zinc-yttrium co-diffusing. The microstructure, the microhardness and the corrosion resistance of the diffusing layer were investigated with optical microscope, SEM, EDS, XRD, microhardness test and the electrochemical corrosion polarization curves test. The results show that the dense diffusing layer with an thickness of 900 μm has formed on the surface of pure magnesium after surface modification treatment. The diffusion layer contains a transition layer and an alloy reaction layer. The transition layer is α-Mg solid solution. The alloy reaction layer contains a plenty of Mg7Zn3, α-Mg solid solution and α-Mg+MgZn lamellar eutectic structure. The microhardness of the surface diffusion alloying layer increases significantly. The average microhardness is about 4 times higher than that of magnesium matrix. The open circuit potential of the alloyed specimen increases by 87 mV. The corrosion current density of alloyed specimen decreases by one order of magnitude. Its corrosion resistance is improved obviously