The　rapid　start-up　and　granulation　of　a　single-stage　partial　nitritation　anammox　granular　sludge　(PN/AnGS)　system　under　limited　seed　sludge　conditions　is　crucial　for　its　practical　application.　This　study　proposed　an　aerobic　granular　sludge　(AGS)　-　based　strategy,　enhanced　the　enrichment　of　anammox　bacteria　(AnAOB),　and　shortened　the　start-up　time　of　PN/AnGS　system　by　20.5%.　In　addition,　the　inoculation　of　AGS　can　ensure　the　stable　operation　of　the　system　during　the　selective　sludge　discharge　to　washout　the　flocs.　Microbial　community　structure,　particle　size　distribution,　morphology　results　showed　that　niche　shift　was　the　key　to　promote　the　enrichment　of　AnAOB,　and　AGS　played　a　decisive　role　in　the　particle　characteristics　of　PN/AnGS.　Since　AGS　can　be　directly　obtained　from　full-scale　AGS　wastewater　treatment　plants,　integrating　PN/AnGS　with　AGS　processes　can　transition　wastewater　treatment　from　a　＂linear　economy＂　to　a　＂circular　economy＂,　enhancing　nitrogen　removal　efficiency　and　delivering　significant　economic　and　environmental　benefits.