Abstract: To address issues such as insufficient mechanical performance of cement-based grouting materials and low utilization of industrial solid waste in deep soft rock mining, a novel high-performance fly ash (FA) modified ultrafine cement (UC) composite grouting material (FASC) was developed under the coupling effect of multi-component admixtures (accelerator, water reducer, and expansion agent). The evolution of injectability, setting behavior, mechanical strength, and microstructure of the FASC slurry with varying FA content was systematically investigated using macroscopic performance tests and microscopic analyses including XRD, FTIR, SEM, and TG. The results show that the incorporation of FA prolongs the setting time of FASC, increases fluidity, reduces bleeding rate, and improves slurry stability. The compressive and flexural strengths first increase and then decrease with increasing FA content. FASC with 5% FA content exhibits optimal mechanical properties, achieving a 28-day compressive strength of 39.8 MPa (a 3% increase compared to the reference), and 1-day and 28-day flexural strengths increased by 12.6% and 8.7%, respectively. Microscopic analysis reveals that an appropriate amount of FA consumes Ca(OH)? through the pozzolanic effect, promotes the formation of secondary C-S-H gel, optimizes the pore structure, and densifies the matrix. Sandstone aggregate cementation reinforcement tests demonstrate that 5% FA enhances the compressive strength and elastic modulus of the ultrafine cement-based cemented body by 13.1% and 9.2%, respectively, along with improved deformation recovery capacity. This enhancement is primarily attributed to the synergistic coupling effect of FA on FASC, characterized by "physical filling – chemical bonding – crack passivation".