The　two-stage　partial　nitrification　(PN)-Anammox　process,　during　long　term　treatment　of　high-ammonia　nitrogen　leachate,　faces　challenges　such　as　the　adaptation　of　nitrite　oxidation　bacteria　(NOB)　and　failure　of　real-time　control　of　pH.　Resultant　instabilities　including　NH4+-N　and　NO3--N　accumulation　were　overcome　by　culturing　sludge　fermentation　liquid　(SFL)-driven　partial　denitrification　(PD)　in　situ　in　the　Anammox　process.　Biodegradation　of　slowly　biodegradable　organics　(SBO)　in　SFL　created　organics　restriction　condition,　which　limited　the　activity　of　denitrification　bacteria　and　achieved　its　balance　with　Anammox　bacteria.　Produced　NO3--N　is　reduced　to　NO2--N　through　PD,　which　further　improved　the　removal　of　NH4+-N　through　Anammox.　NO2--N　was　utilized　timely　by　Anammox　bacteria,　which　avoid　further　reduction　of　NO2--N　to　N2,　and　result　in　a　high　nitrate　to　nitrite　transformation　ratio　(NTR)　of　93.3%.　Satisfactory　nitrogen　removal　efficiency　(NRE)　and　nitrogen　removal　rate　(NRR)　of　99.6%　and　822.0 ± 9.0 g N/(m3∙d)　were　obtained,　respectively.　Key　genera　related　to　degradation　of　SBO,　PD　and　Anammox　were　enriched.　The　value　of　narG/(nirK+nirS)　increased　from　0.05　on　day　1-0.15　on　day　250.　Combining　SFL-driven　PD　with　two-stage　Anammox　process　provided　a　novel　insight　for　applying　this　process　to　realize　advanced　nitrogen　removal　in　practical　engineering.