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.