<p>Fluctuations in pollutant removal efficiency remain a major challenge in sequencing batch reactors (SBRs), restricting their stability in wastewater treatment applications. Bioaugmentation with functional microbial agents has emerged as a promising approach to improve system performance. In this study, ONP-3 attapulgite-based mixed immobilized microbial agents (ONP-3 agents) were prepared by immobilizing the denitrifying strain FX-4, the phosphorus-removing strain CP-7, and the oil-degrading strain YZ-1 using attapulgite as a carrier. The ONP-3 agents were added at different dosages into six parallel SBRs to assess their effects on pollutant removal and operational stability. Results demonstrated that the bioaugmented SBRs achieved significantly higher pollutant removal efficiency than the control group. At an optimal dosage of 0.2 g/L, the removal efficiencies of key pollutants such as COD and TN increased by approximately 5% compared to the control. The addition of ONP-3 agents also stimulated the production of key quorum sensing signaling molecules, C6-HSL and C8-HSL, which promoted extracellular polymeric substance (EPS) synthesis. Specifically, protein content in EPS increased up to 155 mg/g VSS, while the PN/PS ratio remained stable, thereby supporting the robustness of activated sludge in pollutant removal. Moreover, reactor performance exhibited a significant positive correlation with the relative abundance of Pseudomonas (the dominant genus within the ONP-3 agents) (<i>P</i> ≤ 0.01). Overall, this study demonstrates that immobilized microbial agents can effectively enhance pollutant removal efficiency and stability in SBRs. These findings provide a theoretical basis for the development of optimized bioaugmentation strategies in biological wastewater treatment systems.</p>