Abstract: High-water-content materials are a new type of inorganic cementitious material. They have been widely used in the mining of underground mined-out areas in recent years. However, the higher filling cost has always been the key limiting factor in their further development and application. Meanwhile, large volumes of industrial waste, such as fly ash, has become a serious environmental problem as well as a wasted resource, at a time when the rational repurposing of industrial waste is of great significance to the developed world. To solve the problems of the high cost of mine-filling material, the wasting of a useful resource, and the environmental pollution caused by large surpluses of industrial waste like fly ash, the physical and mechanical properties, microstructure, and chemical components of high-water-content materials of varying fly ash content were studied. An engineering test model (ETM) mechanics test system, scanning electron microscopy (SEM) scanning device, X-ray diffraction (XRD) diffraction analyzer, and a fly ash modification mechanism were discussed based on the microscopic and phase analysis results. The test results show that:(1) with increasing fly ash content, the setting time of high-water-content materials gradually increases, water content decreases, and bulk remained relatively unchangs; (2) high-water-content materials, with or without fly ash, are elastoplastic materials, and their deformation and failure progress could be divided into pore compaction stage, elastic stage, yield stage, and failure stage; (3) the peak strength, elastic modulus, and deformation modulus of high-water-content materials are reduced with increasing fly ash content, although residual strength is improved; and (4) the most reasonable dosage of fly ash is 15% when strength, modulus and cost are considered. Peak intensity, elastic modulus, and deformation modulus of high-water-content material are reduced by only 25%, 8.6% and 10% at this fly ash dosage, respectively, and the residual strength increased by 50%. Phase and morphology analyses show that the amount of fly ash affects the hydration progress of β-C₂S, resulting in reduction of ettringite and an increase in other hydration products. Thus, the homogeneity and integrity of the structure of ettringite are destroyed at different levels, leading, eventually, to a reduction in the compressive strength of high-water-content materials.