Autotrophic anaerobic ammonium oxidation coupled to Fe(III) reduction (Feammox) is a promising technology for treating low C/N wastewater. However, Feammox still faces bottlenecks of slow ammonium oxidation rate and the continuous supply of Fe(III) source. This study adopts micro-oxygen strategy to overcome these obstacles. Micro-oxygen increased the ammonium oxidation rate up to 5.7 times higher than under anaerobic condition, and drove the iron cycle in the form of vivianite [Fe(II)] and leucophosphite [Fe(III)]. Furthermore, it was confirmed that the ammonium oxidation in Feammox relies on ammonia monooxygenase (AMO), as evidenced by 10 times increase in the relative amo expression and 1.2 times increase in AMO activity under micro-oxygen compared to anaerobic condition. Additionally, this approach enhanced the growth and co-metabolism of functional bacteria. Long-term experiments demonstrated the sustainability of the Feammox system with iron cycle under micro-oxygen condition. These findings provide valuable insights into the practical application of Feammox process.