Alkali residue, a solid waste with potential cementitious properties, poses severe environmental risks when piled in a large amount. To enable large-scale utilization of alkali residue, this study partially replaced cement with alkali residue to prepare alkali residue modified cemented backfill. Dynamic impact tests were conducted to investigate mechanical behavior and energy evolution characteristics of alkali residue modified cemented backfill. Results indicated that incorporation of alkali residue altered the mechanical performance and enhanced compressive strength of the backfill. With increasing alkali residue replacement proportion, compressive strength first increased and then decreased, peaking at an alkali residue replacement proportion of 5%. Energy evolution of alkali residue modified backfill can be categorized into three stages: linear elastic stage, plastic yield stage, and instable failure propagation stage. Addition of alkali residue brought elastic strain energy closer to total input energy during the first stage, reduced instances of negative dissipated energy, and rendered the energy curves more physically realistic. As alkali residue content increased, the maximum elastic strain energy initially rose and then declined, reaching its peak at an alkali residue replacement proportion of 5%. These findings demonstrated that alkali residue improved mechanical performance of cemented backfill, thereby providing a theoretical basis for the application of alkali residue in mine backfilling operations.