Abstract: With the advancement of the national “deep-ocean and deep-earth” strategy in China, underground space engineering projects, typically represented by plateau railway construction, mineral resource development, oil and gas extraction and utilization, and water conservancy and hydropower construction, are under construction on a large scale. In these projects, high-temperature inrush water temperatures can reach 92.6 ℃, posing a serious threat to the safe and efficient construction of deep underground projects and the lives of construction workers. 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. The results show that under standard curing temperature (20 ℃), the setting time of MK-cement-based grout gradually increases with increasing MK dosage, whereas the opposite trend occurs in high-temperature environments (40 ℃–95 ℃), indicating that MK can shorten the setting time of high-temperature grout. MK-cement-based grout conforms to the Herschel–Bulkley flow pattern and exhibits shear-thinning characteristics. With MK content increasing, the yield stress of the high-temperature grout gradually increases, which has a positive effect on preventing grout loss during grouting treatment of high-temperature sudden water inrush disasters. At 40 ℃ and 80 ℃, 6% MK can reduce the apparent viscosity of the grout, which is beneficial for mixing and pumping in practical engineering. At 40 ℃ and 80 ℃, a 6% MK content reduces the apparent viscosity of the slurry, improving high-temperature pumpability. Within the 20 ℃–80 ℃ range, the addition of MK increases the high-temperature compressive strength of the grout stone and reduces the strength reduction at different curing ages. Specifically, 8% MK increases the three-day compressive strength of the grout stone at 60 ℃ by 89.3%, reduces the 28 day strength reduction at 60 ℃ by 24.4%, and reduces the 28 day strength reduction at 80 ℃ by 16.7%. MK can increase secondary hydration reactions with cement hydration products to generate hydrated calcium silicate and hydrated calcium aluminosilicate, improving cement performance. Under high-temperature conditions, with increasing MK dosage, the content and overlap structure of hydration products and secondary hydration products change; the internal density of the cementitious aggregates initially improves and then decreases. At 95 ℃, 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. This research provides a scientific basis for the development of high-temperature water grouting materials for deep underground engineering.