During the operation of deep geological repository, the bentonite as a buffer material would be in a high temperature-strong alkaline environment for a long time, and its physical properties would 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 ℃ and C_NaOH=0.5 mol/L for duration of 0, 15, 30, 45, 60, 90 days, respectively. 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 of bentonite physical properties over time under high temperature and strong alkali conditions was the dissolution of montmorillonite.