Field tests carried out in Shengli oi lfield showed that underground catalytic aquathermolysis can reduce heavy oil viscosity by about 70-80% with a long-term effect, contributing greatly to oil production increase. Experimental results proved that average molecular weight and contents of heavy components and heteroatoms of heavy oil decrease after the treatment and that H/C ratio and the content of light fractions increase due to reactions, such as hydrogenation, ring opening, desulfurization, etc. initiated by the catalyst, high-temperature water, and reservoir minerals. The study results suggest that in-situ catalytic aquathermolysis of heavy oil is technically feasible.