The　chiral　sillenite　Bi25FeO40　exhibits　versatile　properties　including　electro‐optics,　magneto‐optics,　optical　rotation,　and　photorefraction,　showing　great　potential　in　multi‐functional　magneto‐optical　devices.　However,　the　weak　magnetism　of　this　crystal　leads　to　a　poor　magneto‐optical　property,　which　is　extremely　hindering　its　practical　applications.　Combining　with　the　sillenite　structure,　the　formation　energy　and　the　magnetic‐optical　effect　are　calculated　based　on　density　functional　theory.　The　codoping　of　strongly　magnetic　Co　and　Fe　ions　is　predicted　to　enable　high　doping　concentrations.　The　hybridization　between　Bi　6p　orbitals　and　Fe/Co　3d　orbitals　significantly　enhances　the　optical　transition　and　thus　greatly　improves　the　magneto‐optical　properties.　The　millimeter‐sized　Bi26‐x‐yCoxFeyO40　crystals　are　successfully　prepared　by　hydrothermal　methods.　The　characterizations　on　structure,　TG‐DSC,　magnetism,　and　magneto‐optical　effect　confirm　the　theoretical　predictions　that　the　Co‐Fe　codoped　crystals　maintain　the　chiral　bismuth　ferrite　structure　with　multifunctional　properties　and　high　thermal　stability.　The　Co‐Fe　codoping　significantly　enhances　the　magnetism　and　magneto‐optical　properties　of　Bi25FeO40　crystal.　The　magnetization　of　Bi19.0Co3.5Fe3.5O40　increases　by　27　times　and　the　magneto‐optical　effect　of　Bi19.8Co3.0Fe3.2O40　at　720,　777,　1288,　1388,　and　1790 nm　are　greater　than　Bi:YIG　and　undoped　Bi25FeO40.　Bi26‐x‐yCoxFeyO40　crystals　with　enhanced　magneto‐optical　properties　by　Co‐Fe　codoping　are　promising　for　novel　multifunctional　magneto‐optical　devices.