Anaerobic digestate from organic waste typically exhibits high concentrations of chemical oxygen demand (COD), phosphate and ammonia nitrogen, classifying it as recalcitrant high-strength organic wastewater. This study proposes supplementing digestate with sulfate as an electron acceptor for sulfate-reducing bacteria, aiming to rapidly degrade organic matter while inducing biomineralization to fix carbon, nitrogen, and phosphorus. To investigate the influence of magnesium and calcium ions on COD removal, biomineralization products, and carbon fixation in anaerobic digestate, batch biomineralization experiments were conducted using anaerobic systems with varying magnesium and calcium ion concentrations. The results show that under a total salinity of 10 g/L and a Mg²⁺/Ca²⁺ molar ratio ranging from 0.5 to 10, increasing the Mg²⁺ concentration from 0.12 g/L to 2.4 g/L boosted total organic carbon (TOC) degradation rates initially, but then caused them to decline. The optimal degradation rate was achieved at a Mg²⁺/Ca²⁺ molar ratio of 5, corresponding to 1.2 g/L of Mg²⁺. Furthermore, when the Mg²⁺/Ca²⁺ molar ratio was less than 1, calcite and struvite were the primary mineralization products, whereas monohydrocalcite and struvite prevailed for ratios above 1. These findings suggest that Ca²⁺ mainly facilitates carbon capture, while Mg²⁺ is crucial for immobilizing nitrogen and phosphorus. Consequently, the Mg²⁺/Ca²⁺ molar ratio serves as a key regulator of controlling both the types and proportions of carbonate and phosphate phases.