We developed a new type of catalyst, dimethylbenzenesulfonic copper, composed by hydrogen-donating ligands and a Cu2+ center, for catalytic aquathermolysis viscosity reduction of extra-heavy oil. The component variations of extra-heavy oil before and after aquathermolysis are designedly analyzed via elemental analysis (EL), proton nuclear magnetic resonance (H-1 NMR), and gas chromatography-mass spectrometry (GC-MS). An optimal viscosity reduction value of 81.47% can be achieved using 0.3 wt % catalyst at 240 degrees C for 24 h, where 8.28 wt % of the heavy components, namely, resins and asphaltenes, are converted to light saturates and aromatics. The improved viscosity reduction efficiency can be ascribed to the hydrogen-donating role from the dimethylbenzenesulfonic ligands. The active hydrogens, not only generated from these ligands but originated from the dehydrogenation of naphthene aromatics during the aquathermolysis, are capable of tackling the heavy components or participating in the hydrogenation of heavy components for heavy-light conversion, leading to an enhanced visbreaking effect and improved oil quality. The results enrich the catalytic mechanism of aquathermolysis, which can guide the design and development of new efficient catalysts.