Abstract: To address the poor pumpability and reduced mechanical strength of conventional single-liquid cement slurries at high temperatures, the effect of metakaolin (MK) on the high-temperature slurry and mechanical properties of grout stone of cement-based materials was investigated. Results show that, within the 20°C-95°C range, the addition of MK increases the slurry's yield stress, positively impacting slurry loss during grouting to manage high-temperature water inrush hazards. At 40°C and 80°C, a 6% MK content reduces the slurry's apparent viscosity, improving high-temperature pumpability. Within the 20°C-80°C range, the addition of MK effectively increases the high-temperature compressive strength of the grout stone and reduces the strength reduction at different curing ages. Specifically, 8% MK increases the 3-day compressive strength of the grout stone at 60°C by 89.3%, reduces the 28-day strength reduction at 60°C by 24.4%, and reduces the 28-day strength reduction at 80°C by 16.7%. At 95°C, the addition of MK actually reduces the high-temperature compressive strength of the grout stone. A combination of microscopic testing methods (infrared spectroscopy, X-ray diffraction, low-field nuclear magnetic resonance, and scanning electron microscopy) revealed the mechanism by which MK affects the high-temperature performance of cement-based grouting materials. On the one hand, the small particle size, large specific surface area, and strong water absorption capacity of MK particles reduce the number of free water molecules in the slurry, reducing the thickness of the water film wrapped around the particle surface, increasing the frictional resistance between solid particles, and strengthening the resistance to slurry flow, thereby increasing the yield stress. On the other hand, the surface nucleation effect of metakaolin provides a good nucleation site for high-temperature cement hydration, improving the degree of hydration. Its high pozzolanic effect promotes the secondary reaction of the cement hydration product, Ca(OH)2, at high temperatures, optimizing the micropore structure of the grout stone and enhancing its mechanical strength. This research provides a scientific basis for the development of high-temperature water grouting materials for deep underground engineering.