Display omitted •Coal gangue-construction solid waste backfilling materials with different ratios were designed.•The addition of construction solid waste is negatively correlated with the strength of backfilling materials.•The CH component in the material exhibited a trend of initially increasing and then decreasing.•In the middle stage of hydration, fly ash and activators enhanced the backfilling performance. In order to effectively address the issues of coal gangue emissions and the accumulation of construction solid waste, a combined treatment of coal gangue and construction solid waste can be implemented. In this study, a composite backfilling material was designed, with coal gangue and construction solid waste as the main aggregates. Through laboratory experiments, the time-dependent flow performance, mechanical properties, and microstructure of the material were analyzed under different proportions of factors including fly ash content, substitution rate of construction solid waste, mass concentration, and water reducing agent content. The research results indicate the following: The effects of reducing agent content, mass concentration, fly ash content, and substitution rate of construction solid waste on flow performance increased in that order. The addition of construction solid waste is negatively correlated with the strength of filling materials. The appropriate substitution ratios for FA, construction solid waste, mass concentration, and reducing agent were determined to be 21 %, 50 %, 80 %, and 0.6 %, respectively. The CH component in the material exhibited a trend of initially increasing and then decreasing, providing a large amount of reactants for secondary hydration reactions and strengthening the internal structure of the material. In the middle stage of hydration, FA and activators played an important role in the later stage by providing raw materials for secondary hydration, enriching the internal composition of the structure, and enhancing the backfilling performance of the backfilling material. This study can achieve the recycling and utilization of coal gangue and construction solid waste, and enhance the stability and bearing capacity of backfilling materials.