Abstract: To investigate deterioration effect on thermal conductivity of bentonite as a buffer material in hype-temperature conditions, the bentonite powder was pretreated at different thermal aging temperatures (T=25, 100, 150, 200℃). The thermal conductivity of compacted bentonite samples after pretreatment was measured by thermal probe method, and the effect of thermal aging temperature T on thermal conductivity λ was discussed. Based on laser particle size analysis, X-ray diffraction and thermogravimetric analysis, the microscopic mechanism of the effect of thermal aging temperature T on the thermal conductivity λ of bentonite samples was revealed. The results show that the thermal conductivity λ of bentonite decreases gradually with the increase of thermal aging temperature T, and the maximum decrease is 6.99%. Moreover, the deterioration effect of λ was more significant under high temperatures (100~200℃). The essential reason for the aforementioned changes is that when the T is lower than 100℃ (25~100℃), the temperature effect is predominantly manifested through the desorption of various forms of water, the thinning of the bound water film, the reduction of the particle size of bentonite, resulting in the reduction of the solid phase volume and the increase of the gas phase volume in the sample, which leads to the thermal conductivity λ decreases. When the T is higher than 100℃ (100~200℃), the above temperature effect is further intensified, and the high temperature causes the part of montmorillonite to transform into mica sodium with relatively low λ, resulting in a more obvious reduction of λ in the sample.