Controlled low-strength material (CLSM), known for its self-leveling and self-compacting properties, is widely applied in backfilling projects. However, conventional cement-treated waste soil-based CLSM suffers from poor mechanical performance underwater, limited durability, and inadequate heavy metal immobilization. A modified underwater anti-dispersion CLSM was developed by incorporating ground granulated blast furnace slag to enhance the binder and adding anti-dispersion agents. The durability and heavy metal immobilization performance during underwater backfilling were systematically evaluated. Results show that the proposed CLSM exhibits the highest compressive strength and maintains 88% of its land-to-underwater strength ratio at 28 days. After 12 wet-dry cycles, the strength retention reaches 77.4%, with mass loss controlled within 3%, indicating significantly improved durability over conventional CLSM. During underwater backfilling, the concentrations of heavy metals in the surrounding water are significantly reduced. For instance, the concentration of As decreases by 80.6%, while the leachate from the solidified samples exhibits low levels of contaminants, with Cd concentrations reaching as low as 0.001 mg/L. These findings confirm that the developed CLSM possesses both excellent durability and environmental performance. The developed CLSM provides theoretical and technical support for the resource utilization of construction waste soil in underwater engineering applications.