To research the stability of Mg-based multilayers,two groups of multilayers, ［Mg/Co］20 and ［Mg/SiC］20 were coated on a Si(100) substrate by magnetron sputtering method. The two samples were tested by a microscopy, a surface roughness experiment and an X-ray grazing incidence reflectivity test at room temperature and a relative humidity of 80%.The contract experiments on multilayer damage for Mg/Co和Mg/SiC were performed in the same condition. The results show that after being exposed in the atmospheric environment for 4 days, the damaged area and surface roughness of the Mg/SiC multilayer sample are up to 26.34% and 10 nm, respectively, while those of the Mg/Co multilayer sample are 2.78% and 5 nm, respectively. After 6 days, the 1st reflection peak of Mg/SiC sample is completely disappeared, and Mg/Co sample still has a peak of 47.63% reflectivity. The experimental results show that the Mg/Co multilayer sample has a better environmental stability as compared with the Mg/SiC multilayer. The X Photoelectron Spectroscopy (XPS) shows that the productions of the damaged Mg-based multilayers are mainly MgCO3, Mg(OH)2 and modest MgO. Moreover, the content ratio of Mg (OH)2 and MgCO3 in the inner layer is significantly higher than that in the surface layer. Experimental results suggest that the reason for Mg-based multilayer damage is the erosion of H2O(g), so the capping layer of Mg-based multilayer should prevent the H2O(g) penetrating into the mulilayer.