The　development　of　low-cost　RE-Fe-B　sintered　magnets　with　large　La/Ce　content　is　of　great　significance　for　the　balanced　utilization　of　rare　earth　(RE)　resources,　but　it　is　limited　by　reduced　magnetic　properties.　In　this　work,　the　coercivity　(Hcj),　remanence　(Br),　maximum　energy　product　[(BH)max],　and　temperature　stability　are　simultaneously　enhanced　for　magnets　with　LaCe　accounting　for　40 wt%　of　the　total　RE.　The　synergistic　regulation　of　the　REFe2　phase,　Ce-valence,　and　grain　boundaries　(GBs)　in　RE-Fe-B　sintered　magnets　is　realized　for　the　first　time　by　introducing　appropriate　La　elements.　The　La　elements　inhibit　the　generation　of　the　REFe2　phase　and　tend　to　stay　in　the　triple　junctions,　promoting　the　segregation　of　the　RE/Cu/Ga　elements　and　contributing　to　the　formation　of　Ce/Nd/Cu/Ga-rich　continuous　thicker　lamellar　GBs,　and　as　a　result,　weakening　the　detrimental　effect　on　HA　caused　by　La　element　substitution　and　enhancing　Hcj.　In　addition,　partial　La　atoms　entering　the　RE2Fe14B　phase　are　beneficial　for　improving　the　Br　and　temperature　stability　of　the　magnets　and　promoting　the　Ce3+　ion　ratio,　which　also　provides　additional　benefit　for　Br.　The　findings　provide　an　effective　and　feasible　way　to　co-enhance　the　remanence　and　coercivity　of　RE-Fe-B　sintered　magnets　with　high　Ce　content.　©　2023　The　Authors.　Advanced　Science　published　by　Wiley-VCH　GmbH.