Abstract: To address the challenges of poor flowability, insufficient strength, and high cost in ultra-fine tailings paste backfill, a composite cementitious system composed of CSP (Cemented Solidifying Powder), cement, and ultra-fine tailings was developed. Through systematic mix proportioning and process optimization, the synergistic enhancement of performance and cost efficiency was achieved. The effects of the composite binder system on the rheological properties, slump, and uniaxial compressive strength (UCS) of the paste slurry were experimentally investigated, while the transport characteristics within the pipeline was analyzed using Fluent.The results indicate that the incorporation of cement improves particle size distribution and packing structure, significantly reducing the yield stress and viscosity of the slurry and thereby enhancing flowability, albeit at the expense of strength. Numerical simulations further reveal that increasing cement content elevates the central velocity of the pipeline flow and reduces hydraulic resistance, leading to improved transport performance.By integrating rheological, mechanical, and numerical findings, the optimal mix proportion was determined as CSP∶Cement = 1∶1 with a slurry mass concentration of 62%. Under these conditions, a 28-day UCS exceeding 2.5 MPa is achieved, while the backfill cost is reduced by approximately 20%. The proposed composite backfill system demonstrates a favorable balance among high flowability, adequate strength, and economic efficiency, providing an effective technical pathway for the resource utilization of ultra-fine tailings and the development of green mining practices.