This　study　investigated　the　effect　of　organic　matter　on　the　partial　nitrification　(PN)　performance　of　immobilized　filler.　When　PN　filler　was　put　into　an　aerobic　tank　containing　a　large　amount　of　organic　matter　for　long-term　operation,　it　resulted　in　efficient　ammonia　oxidation.　Next,　the　PN　filler　was　removed,　and　a　continuous-flow　PN　reactor　with　artificial　water　distribution　was　constructed　at　laboratory　scale　to　investigate　the　performance.　The　PN　filler　exhibited　good　resistance　to　the　influence　of　organic　matter.　With　the　increase　in　influent　chemical　oxygen　demand/total　ammonia　nitrogen　(COD/TAN)　from　0.04　to　1.25,　the　PN　performance　of　the　immobilized　filler　did　not　change.　When　the　influent　COD/TAN　was　increased　to　3.22,　ammonia-oxidizing　bacteria　(AOB)　activity　could　be　maintained　by　increasing　the　air　supply.　Batch　experiments　revealed　that　the　reactor　achieved　simultaneous　ammonia　nitrogen　(NH4+-N)　and　COD　removal,　which　indicated　that　AOB　in　the　PN　filler　could　compete　better　with　heterotrophs　to　oxidize　NH4+-N　using　dissolved　oxygen　compared　with　traditional　activated　sludge　and　biofilm　methods.　Nitrite　accumulation　rate　remained　above　95　%　throughout　the　reactor＇s　operation.　High-throughput　sequencing　showed　that　AOB　were　always　the　dominant　bacteria　in　the　microbial　community　inside　the　filler,　and　the　high　tolerance　of　PN　filler　to　organic　matter　depended　on　AOB　abundance.　This　study　provides　technical　support　for　achieving　efficient　and　stable　PN　in　COD-containing　wastewater.　©　2024　The　Authors