Abstract: Aiming at the problems of low strength and high brittleness of coal-based solid waste grouting materials, a polyester fiber modified coal gangue-fly ash-cement-based composite grouting system was adopted. Through systematic two-factor factorial experiment combined with microscopic characterization techniques, the regulation mechanism of polyester fiber content (0%～0.5%) and fly ash content (0%～15%) on material fluidity, mechanical properties and microstructure was revealed. The study found that the use of fly ash instead of Portland cement improved the fluidity of the slurry (increased by 10%) and delayed the setting time (final setting extension of 34.4%), but significantly reduced the strength of the material. The incorporation of polyester fiber can effectively inhibit this strength loss. When the fly ash content was ≤10%, the addition of 0.5% polyester fiber restored the compressive strength to the benchmark level. At a fly ash content of 10%, the flexural strength of the material at 3 d and 28 d was increased by 2.0% and 4.4%, respectively, compared with the neat cement group. Microscopic mechanism analysis shows that fly ash changes the composition and pore structure of hydration products, while polyester fiber blocks crack propagation through three-dimensional bridging. The two synergistically optimize the mechanical properties and fracture toughness of the material.