Abstract: High-efficiency in situ biological denitrification constitutes a pivotal component of water body eutrophication control technologies. Conventional biological denitrification techniques frequently encounter challenges in practical applications within natural water bodies, including low efficiency and difficulties in achieving simultaneous processes within the same spatial configuration. This stems from the substantial disparity in carbon and oxygen requirements between the nitrification and denitrification stages. Based on heterotrophic nitrifying-aerobic denitrifying bacteria, slow-release oxygen materials (CaO₂/clay/cement composite) are combined with modified reed straw slow-release carbon and immobilized microorganisms to improve the redox environment of water bodies and enhance the biological aerobic heterotrophic denitrification efficiency through slow-release oxygen and carbon. The results showed that different pretreatment methods (acid, alkali, heat, hydrothermal, Fenton, and pulverization) altered the carbon release and composition of reed straw. Acid-treated straw exhibited relatively lower and more stable carbon release, lower risk of secondary pollution, and a richer organic composition, which was more conducive to the growth and metabolism of denitrifying microorganisms. Using acid-treated straw to load and enrich screened heterotrophic nitrifying-aerobic denitrifying bacteria, combined with a self-developed slow-release oxygen material, laboratory simulations of nitrogen and carbon removal from overlying water revealed that the removal rates of DOC, NH₄⁺-N, NO₃^--N and TN increased to 53.8%, 93.3%, 98.9%, and 72.6%, respectively, with no accumulation of NO₂^--N, significantly enhancing the denitrification effect, while maintaining TOC at a low level. This study provided a viable remediation strategy for nitrogen pollution control in eutrophic waters, possessing both theoretical significance and practical application potential.