Fouling of ultrafiltration (UF) membranes by natural organic matter (NOM) remains a persistent challenge. Herein, far-ultraviolet (Far-UVC) photolysis of chlorine was developed as a pretreatment to solve the membrane fouling problem arising from NOM. The findings demonstrated that Far-UVC/NaClO pretreatment efficiently increased membrane flux while reducing reversible and irreversible fouling resistance by 80.0 % and 47.1 %, individually. Specifically, Cl•, ClO• and HO• were identified by quenching and probing experiments to play key roles in NOM degradation. Interfacial free energy analysis revealed that Far-UVC/NaClO pretreatment markedly enhanced the repulsive force between contaminant and membrane, thus reducing the tendency of membrane pore clogging. Through molecular-level reaction analysis, unsaturated and aromatic compound macromolecules were decomposed into small molecules, thus reducing membrane pore clogging. Mass difference analysis further revealed the degradation pathways of NOM, which mainly included decarboxylation, dealkylation, and oxygen addition reactions. Moreover, chlorine disinfection by-products (Cl-DBPs) such as trichloroacetic acid, dichloroacetic acid, chloral hydrate and trichloromethane were detected following post-chlorination. Subsequent UF process effectively reduced the formation of DBPs by 19.3-34.3 %. This study demonstrates the application potential of Far-UVC-based oxidation technology to mitigate membrane fouling resulting from NOM.