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Recycling waste for soil improvement is a cost-effective strategy to mitigate the energy consumption and carbon emission of traditional stabilization materials (cement, lime, etc.). This paper proposes an eco-friendly geotechnical improvement using recycled polyester fiber (RPF) and red mud to synergistically reinforce volcanic ash. For red mud -reinforced volcanic ash (RV) with five different RPF contents, a series of mechanical tests, water stability tests, microstructure tests and environmental impact tests were performed. Results show that RPF can significantly improve the mechanical performance and water stability of RV. The reinforcement increases with increasing RPF content, and the optimum contents of RPF is 0.9%. For specimens with optimum RPF incorporation, Unconfined compressive strength (UCS), splitting tensile strength are increased by 122%, 180%, respectively. It also allows saturated soil to still maintain higher UCS and the soil does not completely disintegrate in submerged environment. The pH value and toxicity leaching indicators of specimens both satisfy the specification limits and therefore the material has no environmental hazard. TGA, FT-IR and SEM-EDS indicate that the reinforcement mechanism of RPF on RV derives mainly from fiber-matrix interactions, including interface bonding, bridging effect and limiting micro-cracks development. RPF reinforced RV presents favorable engineering performance as a sustainable construction material and can contribute to the win-win objective of environmental conservation and waste recycling. This low-carbon and sustainable soil improvement technology can serve and guide the design of geotechnical engineering in volcanic areas. • Recycled polyester fiber and red mud are used to co-improve volcanic ash. • Mechanical strength and erosion resistance of fiber-reinforced soil are evaluated. • Reinforcement mechanisms are investigated from microstructural perspective. [ABSTRACT FROM AUTHOR]