The　practical　application　of　photocatalytic　technology　in　the　natural　water　body　is　limited　by　the　low　reuse　capability　and　potential　secondary　pollution　risk　of　powder　catalysts.　Consequently,　it　is　highly　desirable　to　develop　the　stable　floating　photocatalytic　composites　for　efficient　alleviating　harmful　algal　blooms.　In　this　study,　the　self-floating　Ag2O/g-C3N4/hydrogel　composites　with　a　porous　three-dimension　(3D)　network　nanostructure　via　the　cross-linking　of　chitosan　and　polyvinyl　alcohol　have　been　synthesized.　The　effects　of　doping　load,　initial　algal　density　and　dissolved　organic　matter　on　Microcystis　aeruginosa　(M.　aeruginosa)　removal　were　investigated　by　Ag2O/g-C3N4/hydrogel　under　visible　light　irradiation.　Among　the　synthesized　photocatalysts,　the　20-Ag2O/g-C3N4/hydrogel　showed　the　highest　adsorption,　good　reusability　performance　and　excellent　photocatalytic　activity　within　4　h　for　the　removal　of　M.　aeruginosa.　Algal　cell　membrane　and　phycobiliprotein　were　severely　damaged　during　the　photocatalytic　process.　The　inactivation　mechanisms　are　attributed　to　the　water-adsorbing　swelling　of　the　hydrogel　and　the　generated　reactive　oxygen　species　(mainly　·O2−　and　·OH)　by　Ag2O/g-C3N4.　Moreover,　the　antioxidant　system　of　the　algae　cells　was　also　collapsed　under　the　effects　of　reactive　oxygen　species.　The　superoxide　dismutase　(SOD)　and　catalase　(CAT)　content　of　algal　cells　decreased　rapidly　after　photocatalytic　reaction　by　Ag2O/g-C3N4/hydrogel,　while　the　malondialdehyde　(MDA)　activity　was　higher　than　that　of　the　control　group.　The　self-floating　Ag2O/g-C3N4/hydrogel　photocatalyst　would　have　real　practical　applications　for　harmful　algae　in　the　fields　of　natural　water　body.　©　2020　Elsevier　B.V.