To address the poor washout resistance and low strength of grouting materials in water-rich soft strata, a novel cement-based grouting material was prepared by partially replacing ordinary Portland cement with silica fume and metakaolin, together with a polycarboxylate superplasticizer and hydroxypropyl methyl cellulose (HPMC). An L16(45) orthogonal experiment was conducted to systematically investigate the effects of silica fume content (0–15%), metakaolin content (0–15%), water-to-cement ratio (0.5–0.75), superplasticizer content (0–0.3%), and HPMC content (0–0.5%) on the fluidity, setting time, washout resistance, porosity, and compressive strength of the grout. Range analysis was used to determine the optimal mix proportion. The results show that the optimal mix proportion (10% silica fume, 10% metakaolin, water-to-cement ratio of 0.5, 0.1% superplasticizer, and 0.5% HPMC, designated as No.11) achieves a 28-day compressive strength of 52.66 MPa, which is 21.8% higher than that of plain cement paste (43.24 MPa), and 56.0% and 34.3% higher than those of the specimens with 10% silica fume alone (33.74 MPa) and 10% metakaolin alone (39.21 MPa), respectively. The fluidity is 37.8 cm, and the dynamic water retention rate reaches 0.892. The addition of HPMC can increase the dynamic water retention rate up to 0.977 (specimen No.14). X?ray diffraction (XRD) and scanning electron microscopy (SEM) analyses reveal that the pozzolanic reactions of silica fume and metakaolin exhibit a time?progressive synergy at different stages of cement hydration, demonstrating a synergistic effect, which promotes the formation of C?S?H gel and pore densification. The developed material exhibits excellent comprehensive performance and can be used for grouting reinforcement in water-rich soft strata and under dynamic water conditions.