A　partial　nitrification　sequencing　batch　reactor　was　operated　to　reveal　mechanisms　behind　nitrite　oxidation　bacteria　(NOB)　acclimatization　in　high-ammonia　wastewater　treatment.　The　influent　NH4+-N　increased　stepwise　from　499.7 ± 4.2 mg/L　to　6994.5 ± 7.5 mg/L　with　initial　free　ammonia　(FA)　concentration　rising　from　37.9 ± 3.2 mg　NH3-N/L　to　715.3 ± 47.3 mg　NH3-N/L,　respectively.　NOB　acclimatized　this　FA　range　with　NO3--N　production　increasing　from　29.2 ± 2.6 mg/L　to　144.1 ± 31.0 mg/L　in　a　cycle,　which　was　caused　by　the　shift　of　dominant　NOB　genus　from　Nitrospira　to　Nitrolencea.　Nitrosomonas　as　ammonia　oxidation　bacteria,　could　sustain　its　activity　of　62.1 ± 0.1 mg　NH4+-N/(gVSS∙L∙h)　under　the　same　condition.　Hydroxylamine　addition　could　be　implemented　as　an　emergency　measure　to　alleviate　NOB　acclimatization　in　short-term　operation.　The　findings　expanded　knowledge　about　NOB　acclimatization　types　and　provided　novel　insights　for　addressing　this　problem　in　a　targeted　way.