This study developed a photocatalytic method using UV/H₂O₂/TiO₂ to degrade κ-carrageenan (KC), reducing its average molecular weight to 1.6 kDa within 2 hours. The method was optimized through HPLC monitoring of KC concentration and molecular weight changes. The UV/H₂O₂/TiO₂ treatment showed a synergistic effect, leading to a more uniform and lower molecular weight degradation compared to other treatments. The degradation process did not significantly alter the sulfate group content but increased reducing sugar and uronic acid content. FT-IR and UV-Vis analyses confirmed the preservation of the sulfate groups and the formation of carboxyl or aldehyde groups, respectively. XRD analysis indicated slight crystallization improvement after 5 hours of degradation. Rheological properties and SEM images revealed a decrease in viscosity and a change in surface morphology. HPLC-MSn analysis identified oligosaccharide fragments with DP ranging from 1 to 9 and degree of sulfation of 1 to 3. The antioxidant activity of KCO was significantly enhanced compared to KC, showing strong scavenging activity against DPPH and ABTS radicals. In vivo experiments using Caenorhabditis elegans demonstrated that KCO improved survival rates under oxidative stress and UV damage. This study provides a green and efficient method for preparing low-molecular-weight carrageenan oligosaccharides with enhanced biological activities.This study developed a photocatalytic method using UV/H₂O₂/TiO₂ to degrade κ-carrageenan (KC), reducing its average molecular weight to 1.6 kDa within 2 hours. The method was optimized through HPLC monitoring of KC concentration and molecular weight changes. The UV/H₂O₂/TiO₂ treatment showed a synergistic effect, leading to a more uniform and lower molecular weight degradation compared to other treatments. The degradation process did not significantly alter the sulfate group content but increased reducing sugar and uronic acid content. FT-IR and UV-Vis analyses confirmed the preservation of the sulfate groups and the formation of carboxyl or aldehyde groups, respectively. XRD analysis indicated slight crystallization improvement after 5 hours of degradation. Rheological properties and SEM images revealed a decrease in viscosity and a change in surface morphology. HPLC-MSn analysis identified oligosaccharide fragments with DP ranging from 1 to 9 and degree of sulfation of 1 to 3. The antioxidant activity of KCO was significantly enhanced compared to KC, showing strong scavenging activity against DPPH and ABTS radicals. In vivo experiments using Caenorhabditis elegans demonstrated that KCO improved survival rates under oxidative stress and UV damage. This study provides a green and efficient method for preparing low-molecular-weight carrageenan oligosaccharides with enhanced biological activities.