Abstract: To investigate the impact of acid mine drainage on the performance of waste rock−cement−fly ash cemented backfill, laboratory experiments were conducted to simulate acidic environments with different pH values (ranging from 3 to 7). This study systematically examined the evolution patterns of the mechanical properties and microstructure evolution of the backfill under acidic conditions. The results indicate that under weakly acidic conditions (pH=6), the backfill exhibits higher uniaxial compressive and splitting tensile strengths. Compared to the neutral pH control group (pH=7), the 28−day compressive strength increased by 9.97%. In contrast, strongly acidic conditions (pH=3) resulted in significant deterioration of mechanical strength. Microstructural analysis reveals that the weakly acidic environment influences backfill performance by altering the composition of hydration products and the pore structure, promoting the formation of C−S−H gel and ettringite (AFt) and optimizing the pore structure. Conversely, the strong acidity environment causes the dissolution of C−S−H gel, an increase in porosity, a higher proportion of harmful pores, and a loosened structure. XPS and XRD analyses further elucidate the chemical mechanisms underlying the evolution of elemental binding energy and hydration products in the acidic environment. The weakly acidic environment (pH=6) can promote the formation of C−S−H gel and AFT, refine the pore structure, and thereby enhance the mechanical properties of backfill. In contrast, the strongly acidic environment (pH=3) lead to gel dissolution, pore coarsening, resulting in significant degradation of strength. This research provides a theoretical basis for the resource utilization of acid mine drainage in backfill mining and offers significant reference value for assessing the long−term stability of backfill in acidic environments.