A nitritation-anammox single-stage autotrophic nitrogen-removal system was started and achieved stable, efficient operation. The startup process occurred in three stages: aerobic granulation, nitritation regulation, and autotrophic nitrogen removal. Granular sludge was successfully incubated after 57 d of acclimation. Through an operating strategy of gradual reduction in organic loading and an increase in ammonia nitrogen loading, stable nitritation was successfully achieved (average NH₄⁺-N/NO₂ ratio maintained at 1.32). Nitritation was coupled with anammox to achieve autotrophic nitrogen removal. The nitrogen loading rate was increased to 0.3 kgN/(m³•d) and the removal rate reached 0.26 kgN/(m³•d) after 50 d of operation. Test results showed that during the autotrophic nitrogen removal stage, anaerobic ammonia-oxidizing bacteria were considerably enriched, and that there was an organic balance between these and ammonia-oxidizing bacteria in the system.