This　study　investigated　the　relationship　between　the　performance　and　the　ecological　features　of　a　one-stage　partial　nitritation　and　anammox　disturbed　by　oxygen.　The　disturbance　caused　an　irreversible　deterioration　of　the　nitrogen　removal　rate　from　0.8　to　0.05　kg　N/(m3∙d)　although　the　anammox　genera　increased　from　1%　to　1.4%.　Meanwhile,　the　richness　and　evenness　reduced　from　455　and　4.00　to　429　and　3.81,　respectively,　following　a　similar　pattern　to　the　community　complexity.　The　community　drifted　and　formed　three　distinct　clusters　during　and　after　the　disturbance.　Furthermore,　234　of　634　operational　taxonomic　units　in　the　community　were　depleted　despite　recovered　diversity　and　complexity　during　long-term　stable　operation.　In　conclusion,　the　ecological　fluctuation　of　the　microbial　community　with　decreasing　resilience　was　the　driving　force　that　fatally　collapsed　the　system　performance.　This　study　suggests　that　ecological　features　are　conducive　to　the　diagnosis,　prediction,　and　optimization　of　a　partial　nitritation　and　anammox　system.　©　2023　Elsevier　Ltd