Adding　organics　in　the　process　of　geopolymer　synthesis　can　combine　the　functional　groups　of　organics　with　the　three-dimensional　structure　of　geopolymer,　thus　changing　the　properties　of　geopolymer　such　as　compressive　strength,　flexural　strength,　and　acid　resistance.　In　this　work,　excellent　mechanical　properties　and　acid　resistance　of　metakaolin-blast　furnance　slag　(Mk-BFS)-based　geopolymer　were　synthesized　by　the　incorporation　of　chitosan.　The　formation　of　sodium-alumino-silicate-hydrate　(N-A-S–H)　gel　and　calium-alumino-silicate-hydrate　(C-A-S–H)　gel　in　geopolymerization　were　characterized　by　29Si　nuclear　magnetic　resonance　(NMR).　At　5%　of　chitosan,　the　compressive　strength　of　Mk-BFS-based　geopolymer　could　reach　to　33.7 MPa.　The　results　could　be　ascribed　to　that　chitosan　reacts　with　geopolymer　to　generate　new　C-O-Si　structure　meanwhile　adhesion　produced　by　the　combination　of　positively　charged　cations　and　negatively　charged　ions　makes　the　structure　of　geopolymer　denser.　After　7 days　of　sulfuric　acid　immersion,　the　reduction　of　compressive　strength　is　less　than　3 MPa,　demonstrating　its　great　acid　resistance.　The　acid　resistance　of　Mk-BFS-based　geopolymer　could　attribute　to　that　the　free　amino　groups　in　chitosan　preferentially　react　with　acid　solution　and　weakened　the　erosion　of　sulfuric　acid.　This　study　optimizes　the　compressive　strength　and　acid　resistance　of　geopolymer　by　adding　appropriate　amount　of　chitosan.　©　2023,　The　Author(s),　under　exclusive　licence　to　Springer-Verlag　GmbH　Germany,　part　of　Springer　Nature.