Abstract: The backfill technology using paste-like materials boasts advantages of excellent fluidity and high strength, ensuring long-distance transportation and pressure-bearing capacity, thereby facilitating its rapid development in the coal mining industry. However, its widespread application is hindered by the reliance on costly cement as a binder. To reduce costs, current research explores the utilization of inexpensive biomass power plant ash, processed through hydrothermal synthesis and low-temperature calcination, to fabricate a novel cementitious material as a substitute for cement in backfilling. X-ray diffraction and scanning electron microscopy are employed to analyze the hydrothermally synthesized precursors and the low-temperature calcined clinker, investigating the effects of hydrothermal synthesis and low-temperature calcination on the phase development and microstructure of the novel cementitious material. Orthogonal experimental design is adopted to systematically study the impact of various factors and their levels on the properties of the cementitious material, identifying the optimal combination of factors for the hydrothermal synthesis-low temperature calcination process. The feasibility of applying this optimally synthesized novel cementitious material in backfilling is further examined. The results indicate that hydrothermal synthesis significantly accelerates the reaction rate, and the precursor obtained via hydrothermal synthesis more effectively transforms into active substances under low-temperature calcination. Optimal performance is achieved when the Ca/Si moral ratio of raw materials is 1.5, the hydrothermal synthesis temperature is 90 °C, the duration of hydrothermal synthesis is 3 hours, and the calcination temperature is 850 °C. Backfill material prepared with a mass ratio of gangue: fly ash: novel cementitious material: water as 5:3:2:2, using the optimally synthesized cementitious material, meet the requirements for compressive strength and flowability. This study concludes that the novel cementitious material prepared from biomass power plant ash via hydrothermal synthesis and low-temperature calcination can replace cement in backfill materials, reducing the cost of binders in backfill materials. This not only promotes the adoption of paste-like backfill mining technology but also facilitates the utilization of biomass power plant fly ash as a solid waste resource.