Abstract: High temperatures induced by decay heat in high-level radioactive waste(HLW)repositories significantly affect the sealing performance of construction joints in block barriers sealed with bentonite slurry. Investigating the cracking behavior and mechanisms of modified bentonite slurries under different thermal conditions is therefore essential for evaluating the long-term sealing performance of repositories. This study examined bentonite slurries modified with nano-bentonite at contents of 10%, 15%, and 20%. Drying experiments were conducted at 25℃, 45℃, 65℃, and 85℃. We analyzed drying shrinkage using mass-loss measurements and PCAS image processing, and quantified key parameters including evaporation rate, surface crack density, and crack ratio. These results allowed us to evaluate cracking characteristics and clarify the influence mechanism of the modifier on drying shrinkage behavior. We further investigated the action mechanism of nano-bentonite at different temperatures using scanning electron microscopy(SEM). The results showed that nano-bentonite and bentonite formed a dense microstructural network that effectively suppressed moisture evaporation and crack development. As temperature increased, a hardened crust developed on the slurry surface and inhibited rapid water loss, causing the evaporation-rate curve to evolve from a unimodal to a bimodal pattern. Meanwhile, increasing the nano-bentonite content monotonically reduced crack density. The drying process of nano-bentonite-modified bentonite slurry proceeded through four successive stages: initial evaporation, steady evaporation, reduced evaporation, and residual evaporation. Based on a multi-parameter evaluation of slurry viscosity, evaporation rate, and crack density, the bentonite slurry modified with 10% nano-bentonite exhibited the best resistance to drying shrinkage.