Abstract: The water-cement ratio is a key factor affecting the performance of cement-based grouting materials. The hydration characteristics of multi-component cementitious systems at different water-cement ratios exhibit significant differences. To clarify the intrinsic relationship between the water-cement ratio and the performance of mine ultra-fine cement systems, ultra-fine cement-based grouting materials (UPCGM) were prepared by incorporating superplasticizers, expanding agents, and accelerators into ultra-fine silicate cement. Using various macro- and micro-testing methods, the effects of the water-cement ratio (0.45-0.80) on the performance of UPCGM slurry, mechanical properties, and microstructure of the hardened body were explored, and the underlying hydration mechanisms were analyzed in depth. The results indicated: ①The water-cement ratio was positively correlated with the slurry flowability and setting time of UPCGM, while it was negatively correlated with the volume expansion rate of the hardened body. ②As the water-cement ratio increased, the mechanical strength of UPCGM initially increased and then decreased. At a water-cement ratio of 0.50, the hardened body exhibited the best mechanical performance, with a compressive strength of 33.70 MPa at 3 days, an increase of 298.82% to 466.39% compared to water-cement ratios of 0.60 to 0.80. The 28 days compressive strength (50.70 MPa) and flexural strength (8.50 MPa) increased by 18.18% to 85.71% and 50.44% to 95.85%, respectively. ③Thermodynamic modeling, XRD, FTIR, and SEM confirmed that changes in the water-cement ratio affected the hydration degree, hydration products, pore structure, and compactness of UPCGM, with the content and size of AFt increasing as the water-cement ratio increased. ④Compared with the mine inorganic reinforced composite mortar (KWJG-1), UPCGM showed slightly lower flowability, early compressive strength, and flexural strength, but had significantly enhanced slurry stability, early setting properties, and later compressive strength. At water-cement ratios of 0.50 and 0.80, the 28 days compressive strength was 14.50% and 35.48% higher than that of KWJG-1, respectively, indicating potential applications in the field of roadway surrounding rock grouting reinforcement.