<p>The photocatalytic efficiency of organic conjugated polymers is frequently limited by the short lifetime of reactive oxygen intermediates. To address this, we employ molecular side-chain engineering by improving the electron-donating ability of the donor units of donor-acceptor conjugated polymers. The enhanced electron-donating capability of the side chains raises the electron density on the polymer backbone, which in turn accelerates more efficient electron transfer to adsorbed oxygen and anthraquinone units. This process thus extends the lifetime of the key oxygen-centered organic radical (OCORs) intermediates from 20 min in C8L-AQ to 35 min in C8B-AQ, leading to a high solar-to-chemical conversion efficiency of 0.98%. Under natural conditions (air, 100 mW cm⁻², no sacrificial agents), the optimal polymer C8B-AQ achieves remarkable H₂O₂ production rates of 3661 and 6236 µmol g⁻¹ h⁻¹ in pure and lake water, respectively, surpassing most reported organic conjugated photocatalysts. This work establishes a scalable design principle for high-performance polymer photocatalysts in solar-driven small-molecule transformations.</p>