The removal of nitrogen from the mature leachate landfills is a major challenge. In this study, a novel aeration control strategy combined with step-wise influent addition was used to successfully implement simultaneous partial nitrification, anaerobic ammonia oxidation and denitrification (SNAD) in a single-stage Sequencing Biofilm Batch Reactor (SBBR) used for nitrogen removal and carbon removal of mature landfill leachate with a low C/N ratio. The SNAD process was rapidly initiated by transitioning from aeration to aeration-anaerobic (OA) phases and stabilized under anaerobic-aeration-anoxic (AOA) conditions. The relative abundance of nitrite-oxidizing bacteria (NOB) in the reactor was consistently suppressed (<0.1 %), while the dominant genus of anaerobic ammonium-oxidizing bacteria (AnAOB), Candidatus Jettenia, reached a relative abundance of 10.9 % in the biofilm. During 160 days of operation with influent NH4+-N up to 600.9 mg/L and COD/TIN of 2.3 ± 0.3, the system achieved 98.3 ± 1.1 % nitrogen removal efficiency and effluent TIN of 10.8 ± 1.3 mg N/L, demonstrating robust synergy among ammonia-oxidizing bacteria (AOB), anammox bacteria, and denitrifying bacteria (DNB). This strategy provides a scalable approach for SNAD system cultivation and sustainable treatment of mature landfill leachate.