In　this　study,　a　novel　start-up　strategy　for　single-stage　partial　nitritation-anammox　(SPNA)　process　was　developed　in　a　plug-flow　granular　sludge　reactor.　The　activated　sludge　inoculum　and　biofilm-fixing　carriers　were　simultaneously　added　into　reactor　firstly　to　start　up　as　integrated　fixed-film　activated　sludge　(IFAS)　process　with　ammonium　input.　Under　low　dissolved　oxygen　(DO)　concentration　of　3•d)　was　achieved.　As　the　biofilm　was　formed,　matured,　and　detached　repeatedly　during　the　start-up　period,　red-brown　granules　were　observed　in　mixed　suspended　sludge　liquid.　Then　the　media　filling　ratio　was　adjusted　from　20%　to　0%,　resulting　in　a　temporary　decrease　of　granular　size　from　270μm　to　163μm　and　total　inorganic　nitrogen　removal　rate　to　0.85kgN/(m3•d).　However,　the　removal　performance　was　rapidly　recovered　following　the　increment　of　aeration　and　the　average　sludge　particle　size　gradually　increased　by　195μm.The　experimental　results　showed　that　flocs　and　particles　could　be　stable　co-existed　in　the　SPNA　reactor,　and　the　final　loading　rate　of　1.5kgN/(m3•d)　could　be　achieved,　which　was　comparable　to　that　in　IFAS　mode.　Therefore,　the　start-up　method　reported　could　be　an　effective　pathway　to　fulfill　high-rate　autotrophic　nitrogen　removal　in　plug-flow　granular　sludge　systems.　©　2021,　Editorial　Board　of　China　Environmental　Science.　All　right　reserved.