Abstract: Two-stage nitrosation-anaerobic ammonium oxidation process can solve the problem of poor biodegradability of digested effluent and improve the ability of ammonia nitrogen (NH₄⁺-N) and total nitrogen (TN) removal. In this study, the method of gradually increasing the digested effluent in the influent of anammox process was used to domesticate and enrich anammox bacteria. Then, the operating characteristics of the nitrosation and anammox stages in the starter and operation periods were studied using digested effluent as the treatment agent. The anaerobic ammonia oxidation reactor was successfully started up in about 117 days, which greatly shortened the start-up time. Meanwhile, it was found that the ratio of NH₄⁺-N/ NO₂^--N (nitrite nitrogen) in the influent is an important factor affecting the operation performance of the anaerobic ammonia oxidation reactor. The effects of nitrosation and anammox stages on chemical oxygen demand (COD) and nitrogen removal are inconsistent: the nitrosation stage accounted for 82% of the COD removal in the two-stage process, while the anammox stage had almost no effect on the COD removal. From the perspective of N flow direction, the nitrosation section mainly transforms N, but had almost no effect on TN removal. Moreover, the anaerobic ammonia oxidation section played a major role in TN removal. The average TN removal rate is 72.42%, and the contribution of the TN removal rate in the system is 96.40%. During the operation of the two-stage system, the overall average NO₂^--N consumption ratio was close to the theoretical value due to the trace amount of dissolved oxygen, but the lack of a carbon source led to a significant increase in the NO₃^--N production ratio. This, in turn, made the apparent NH₄⁺-N consumption in the anaerobic ammonia oxidation section∶NO₂^--N consumption∶NO₃^--N production =1∶1.34∶0.39, which is different from the theoretical value of 1∶1.32∶0.26.