This study developed the hybrid partial-nitrification and anammox (HPNA) reactor to increase efficiency in ammonia nitrogen (NH₄⁺-N) removal. The HPNA reactor contained a fixed bed filled with biofilm carrier and a fluidized bed with mechanical stirrer to maximize withholding the sludge. The reactor achieved a total nitrogen (TN) removal efficiency of 70% and NH₄⁺-N removal efficiency of 84%. The maximum nitrogen loading rate (NLR) of 6.1 kg-N/m³/d and nitrogen removal rate (NRR) of 4.7 kg-N/m³/d were achieved with a short hydraulic retention time (HRT) of 3 h. After 160 days of operation, both the mixed liquid suspended solids (MLSS) and the mixed liquor volatile suspended solids (MLVSS) increased in the lower part (from 3.7 to 14.7 g/L and 1.8 to 10.9 g/L, respectively) and the upper part (from 3.7 to 11.8 g/L and 1.9 to 6.7 g/L, respectively). The proteins (PN)/ polysaccharides (PS) ratio increased from 0.96 to 1.1 in the lower part but decreased to 0.7 in the upper part. Bacterial community profiles generated from 16S rRNA sequences further showed that the ammonium-oxidizing bacteria (AOB) Nitrosomonas and the anammox bacterium Brocadiales were enriched in the HPNA reactor.