Abstract: High-efficiency in situ biological denitrification plays a vital role in water eutrophication control technologies. Conventional biological denitrification technologies 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 nitrification and denitrification. In this study, heterotrophic nitrifying-aerobic denitrifying bacteria were employed. We combined slow-release oxygen materials (CaO₂/clay/cement composite) with modified reed straw slow-release carbon and immobilized microorganisms to improve the redox environment of water bodies and enhance the aerobic heterotrophic denitrification efficiency through slow-release oxygen and carbon. Subsequently, we compared the carbon release and composition differences of the leachate from reed straw pretreated with various methods: acid, alkali, heat, hydrothermal, Fenton, and pulverization. The denitrification effect of the heterotrophic nitrifying-aerobic denitrifying bacteria in the corresponding leachates was also considered to comprehensively evaluate the pretreatment efficiency. Based on these findings, a slow-release oxygen material-straw immobilized microorganism composite technology was developed and its denitrification efficiency was investigated alongside comparative technologies. The results showed that among various pretreatment methods for straw, acid-treated straw exhibited relatively lower and more stable carbon release, along with lower risk of secondary pollution. Moreover, its leachate had a richer organic composition with a higher proportion of fulvic acid-like components, which was more conducive to the growth and metabolism of denitrifying microorganisms. When this leachate was used as the sole carbon source for the ADM-2 bacterial community, both the \mathrmNO_3^- -N removal efficiency (65.10%) and removal rate (0.06 mg/(L·h)) were the highest. By immobilizing highly efficient microbial communities on straw and coupling them with slow-release oxygen materials, the removal rates of DOC, \mathrmNH_4^+ -N, \mathrmNO_3^- -N and TN in the overlying water increased to 53.8%, 93.3%, 98.9%, and 72.6%, respectively, during simulated remediation, with no accumulation of \mathrmNO_2^- -N. This composite technology significantly enhanced the denitrification effect. DOC levels remained between 0.8 and 1.5 mg/L in the overlying water, meeting carbon source requirements while avoiding the risk of secondary pollution. This composite technology addresses the challenge of simultaneously supplying carbon and oxygen for the efficient matching of nitrification and denitrification during nitrogen removal in natural water bodies, demonstrating practical application potential in the field of efficient in-situ biological denitrification in eutrophic waters.