Extremely　acidic　conditions　(pH　＜　1.0)　during　hydrogen　sulfide　(H2S)　biotreatment　significantly　reduce　the　cost　of　pH　regulation;　however,　there　remain　challenges　to　its　applications.　The　present　study　investigated　the　H2S　removal　and　biomass　variations　in　biotrickling　filter　(BTF)　under　long-term　highly　acidic　conditions.　A　BTF　operated　for　144　days　at　pH　0.5-1.0　achieved　an　H2S　elimination　capacity　(EC)　of　109.9　g/(m(3).h)　(removal　efficiency　=　97.0%)　at　an　empty　bed　retention　time　of　20　s,　with　an　average　biomass　concentration　at　20.6　g/LBTF.　The　biomass　concentration　at　neutral　pH　increased　from　22.3　to　49.5　g/L-BTF　within　28　days.　In　this　case,　elemental　sulfur　(S-0)　accumulated　due　to　insufficient　oxygen　transfer　in　biofilm,　which　aggravated　the　BTF　blockage　problem.　After　long-term　domestication　under　extremely　acidic　conditions,　a　mixotrophic　acidophilic　sulfur-oxidizing　bacteria　(SOB)　Alicyclobacillus　(abundance　55.4%)　were　enriched　in　the　extremely　acidic　biofilm,　while　non-aciduric　bacteria　were　eliminated,　which　maintained　the　balance　of　biofilm　thickness.　Biofilm　with　optimum　thickness　ensured　oxygen　transfer　and　H2S　oxidation,　avoiding　the　accumulation　of　S-0.　The　BTF　performance　improved　due　to　the　enrichment　of　active　mixotrophic　SOB　with　high　abundance　under　extremely　acidic　conditions.　The　mixotrophic　SOB　is　expected　to　be　further　enriched　under　extremely　acidic　conditions　by　adding　carbohydrates　to　enhance　H2S　removal.