During the operation of the deep geological repository, the bentonite as a buffer material will be in a high temperature-strong alkaline environment for a long time, and its physical properties will deteriorate with time. Aging effect on physical property of MX-80 bentonite powder at high temperature and strong alkali conditions was investigated in this paper. MX-80 bentonite was subjected to alkali-heat conditions, i.e.,T=90°C and CNaOH=0.5mol/L for a time duration of 0, 15, 30, 45, 60, 90 days. The variation of physical properties such as the specific gravity, specific surface area and swell index of MX-80 bentonite was studied through laboratory experiments. Some representative specimens were selected for X-ray diffraction (XRD) tests and thermogravimetric analysis (TGA) tests to provide a reasonable explanation from microscopic perspective. The results showed that the specific gravity, specific surface area and swell index of bentonite decreased sharply from 0 to 45 days, gradually levelling off after then, finally, the reduction amplitudes were 13.6%, 18.5% and 58.1% after 90 days, respectively. Montmorillonite content decreased and albite content increased. The changes of montmorillonite and albite were -23.3% and +3.7% after 90 days. The content of free water, weakly bound water and strongly bound water decreased, and the content of the three decreased by 2.84%, 0.13% and 0.24% respectively after 90 days. The fundamental reason for the deterioration effect of bentonite physical properties over time under high temperature and strong alkali conditions was the dissolution of montmorillonite. The decrease in montmorillonite content leaded to a decrease in specific gravity, swell index and specific surface.