Abstract: Large-scale stockpiling of red mud causes serious environmental pollution and potential safety hazards. To realize large-scale underground disposal of red mud,proportion optimization of red-mud-based cemented backfill material was carried out with the background of backfill mining in a water-rich bauxite mine in Guizhou. Orthogonal tests were adopted to investigate the effects of mixture factors on mechanical properties,water stability of backfill and transportation characteristics of filling slurry,and the synergistic strengthening mechanism of red mud-fly ash composite system was revealed. The results show that the strong-alkaline environment provided by red mud can activate the pozzolanic activity of fly ash,and fly ash optimizes the particle gradation of material system. Together they densify the micro-pore structure and improve mechanical and water-resistant performance of backfill. Slurry concentration is the dominant factor governing bleeding property and rheological pumpability,and the mixture range should satisfy both service performance of backfill and pipeline transportation requirements. Regression models under multi-index constraints between performances and mixture variables were established. The mixture scheme adaptive to mine conditions was obtained by optimization with the objective of minimum backfill cost. This study provides theoretical support and engineering reference for resource utilization of high-alkali red mud and backfill mining in waterrich bauxite mines.