Food waste leachate is a high–strength wastewater characterized by refractory organics, high–salinity and elevated ammonium concentrations, posing challenges for effective treatment and nitrogen resource recovery. In this study, a novel strategy integrating UV/PMS advanced oxidation pretreatment with aerobic heterotrophic ammonium assimilation (HAA) was employed to enhance microbial nitrogen assimilation and carbon removal. Long–term monitoring revealed that the UV/PMS–HAA system achieved superior NH₄⁺–N and COD removal efficiencies of 84.04 % and 90.74 % compared to the control. EPS analysis indicated higher protein content and tighter sludge structure, supporting improved microbial aggregation. Microbial diversity was significantly enhanced in the UV/PMS–HAA system, with enrichment of functional genera such as Halomonas, Pseudomonas and Thauera. Network and robustness analysis revealed intensified microbial cooperation and reduced disturbance sensitivity. Substantial upregulation of ammonium assimilation genes (gdhA, glnA, gltB), while nitrification–related genes (amoA, hao) were nearly absent, confirming a heterotrophic assimilation–dominated pathway. Enzyme activity analysis further supported this trend, with elevated GS, GOGAT activity and higher intracellular Glu, Gln, and TAA levels in the UV/PMS–HAA. UV/PMS pretreatment effectively reshaped microbial structure and function, promoting nitrogen recovery through assimilation rather than loss via nitrification, and provides a promising solution for treating complex nitrogen–rich wastewaters.