Abstract: Alteration of simulated high-level waste (HLW) glass in humid vapor at high temperature was investigated to simulate the penetration of groundwater to geological disposal barrier and accelerated hydration of HLW glass. Glass samples were hanged in the autoclaves in a hypoxia cell. Pure water and special saline solution were placed in the autoclave to maintain the relative humidity at 90% and 100%, respectively. The autoclaves were sealed and placed at 120 ℃ and 150 ℃ to maintain the vapor humidity. The concentration of the key elements and corrosion products are analyzed to determine the alteration rate of glass. The results show that the effect of temperature on alteration rate of glass is more remarkable than that of humidity. The leaching rate of most elements in glass at 150 ℃ are 2-3 orders of magnitude higher than those at 120 ℃. There is obvious alteration phenomenon after 90 days at 150 ℃. After 900 days experiment, the thickness of corrosion layers is 800-1 000 μm at 150 ℃, while the corrosion occurrs slightly at 120 ℃ with a thickness less than 50 μm. With the occurrence of alteration at 150 ℃, the SiO₂ corrosion layer on the surface of glass turns to silicate of calcium and forms secondary minerals.