Main observation and conclusionrnGreen plants and algae utilize complicated yet delicate structures to oxidize water into molecular oxygen while releasing electrons for dark reactions.The photocatalytic water oxidation reaction in photosystem Ⅱ is challenging as it involves in transferring multiple elec-trons and protons and requires a large overpotential.Herein,for the first time,we report that conjugated ladder polymers with suita-ble band structures are potential metal-free photocatalysts for efficient and stable water oxidation.Two structurally similar conjugat-ed ladder polymers are synthesized via the condensation reaction between amine-and ketone-containing monomers.Photocatalytic tests reveal that different polymer structures exhibit different photocatalytic performances which are determined by their inherent optoelectronics properties.Meanwhile,our first-principles calculations confirm that photocatalytic water oxidation is thermodynam-ically feasible for both conjugated ladder polymers under visible light irradiation.Our study unveils a novel class of polymer materials for the water oxidation reaction and further indicates that metal-free photocatalysts hold great potential for future solar-to-chemical conversion.