Fractures are widely present in subsurface formations, and fracture extension under ultralow temperature conditions is important for tasks such as liquid nitrogen fracturing and underground storage of liquid natural gas or liquid air. Therefore, the fracture toughness of sandstone under ultralow-temperature conditions was investigated using three-point bending tests of the NSCB specimens. The specimen damage processes were monitored using the DIC technique, and the changes in the pore microscopic characteristics after ultralow-temperature treatment were observed using CT and SEM. The results show that fracture toughness of dry sandstone was insensitive to the variations of the ultralow temperature (-30 to −120 ℃), whereas that of saturated sandstone increased gradually with the decrease of temperature due to the cementing effect of ice. DIC analysis showed that the peak strain around the fracture tip increased, and the fracture extension time was shortened with a decrease in temperature for both dry and saturated sandstone samples. The minerals shrank and the samples became more compacted with a decrease in temperature for the dry samples, resulting in their brittle characteristics under ultralow temperatures. The enhanced cementation effects of ice under ultralow temperatures induced high internal stress and rapid energy release upon failure of the saturated samples. Large pores shrank owing to pore collapse and filling after the one-cycle ultralow-temperature freeze–thaw treatment.