Conjugated　polymers　feature　promising　structure　and　properties　for　photocatalytic　water　splitting.　Herein,　a　hydrolysis　strategy　was　demonstrated　to　rationally　modulate　the　surface　hydrophilicity　and　band　structures　of　conjugated　poly-benzothiadiazoles.　High　hydrophilicity　not　only　enhances　the　dispersions　of　polymeric　solids　in　an　aqueous　solution　but　also　reduces　the　absorption　energy　of　water　molecules.　Besides,　both　theoretical　and　experimental　results　reveal　that　a　more　positive　valence　band　potential　is　generated,　which　contributes　to　enhancing　the　photocatalytic　water　oxidation　performance.　Accordingly,　the　surface-modified　conjugated　polymers　show　largely　promoted　photocatalytic　water　oxidation　activities　by　deposition　of　cobalt　oxides　as　cocatalysts.　©　2022　Wiley-VCH　GmbH.