The　partial-denitrification　coupled　to　anammox　process　(PD/A)　has　been　emerging　as　a　potential　alternative　for　cost-efficient　nitrogen　removal　from　wastewater.　Although　there　has　been　intensive　study　on　startup　and　optimization,　efficient　reactivation　and　controlling　strategy　after　disturbance　of　this　process　still　lack　essential　information.　Here,　a　selective　bioaugmentation　strategy　with　low　dosage　for　stepwise　recovery　of　PD　and　anammox　activity　were　proposed　and　demonstrated　for　the　first　time,　and　the　mechanism　of　coupling　process　reactivation　was　revealed　based　on　metagenomic　analysis.　A　significant　increase　in　nitrite　(NO2-N)　production　was　obtained　after　introducing　PD　inoculum　by　weight　ratio　of　7.7　%,　with　nitrate　(NO3-N)-to-NO2　-N　transformation　ratio　(NTR)　increasing　from　15.7　%　to　45.0　%,　while　anammox　activity　stayed　at　a　relatively　low　level.　The　ammonia　(NH4+-N)　removal　rate　significantly　increased　by　4.0　times　after　supplying　PD/A　biomass　by　only　1.2　%　due　to　sufficient　substrate　NO2-N　for　anammox.　Consequently,　nitrogen　removal　efficiency　was　remarkably　improved　to　99.2　%　and　kept　stable　with　low-strength　wastewater.　Metagenomics　analysis　revealed　that　the　abundances　of　genes　nar　and　nap　encoding　NO3-　-N　reductase　were　kept　at　much　higher　level　than　nir　for　NO2-　-N　reductase.　Significantly,　higher　diversity　and　abundance　of　genes　responsible　for　carbon　metabolism　could　accelerate　the　electron　generation　and　transfer　to　PD,　thus　stimulating　anammox　activity,　which　was　the　key　reason　for　the　rapid　reactivation　of　PD/A　process.　Furthermore,　the　substate　ratio,　nitrogen　loading　rates　and　morphological　characteristics　of　sludge　have　crucial　influence　on　the　recovery　of　biomass　activity.　The　dominant　genus　Thauera　responsible　for　PD　and　Candidatus　Brocadia　for　anammox　coexisted　stably　after　bioaugmentation.　These　results　provide　a　comprehensive　understanding　of　reactivating　and　regulating　a　novel　PD/A　process　towards　application　and　microbial　interaction.