Abstract: Field tests of underground heavy oil catalytic aquathermolysis were carried out in Shengli oilfield, the average period oil increment for 5 test wells reached to 653 t, and the viscosity of heavy oil was reduced by 79.8%, and still decreased by more than 62% after 14 weeks. The chemical and physical properties of heavy oil before and after the reaction were investigated using DV-III Ultra-Brookfield rheometers, Elementar Vario EL III elemental analyzer, Knauer K-700 Vapor permeability tester, Agilent 6890N gas chromatograph and EQUINOX 55 Fourier transform infrared spectrometer,etc. The results indicate that the heavy oil viscosity and average molecular weight are decreased and the content of resin and asphaltene is reduced after the reaction. The H/C ratio of heavy oil and the content of saturate and aromatic are increased. The amount of heteroatom in heavy oil is also decreased after the treatment. The reaction of underground heavy oil catalytic aquathermolysis is mainly affected by the catalyst system, high temperature water and reservoir mineral, in which the catalyst is the key control factor, and the hydrogen donating accelerator and dispersing agent can improve the cracking. In addition, the acid-base properties of water under high temperature and the reservoir mineral can promote the reaction. During the aquathermolysis, many reactions including the removal of alkyl side chain, the molecular chain isomerization, the hydrogenation, the ring opening, the ring closing and the desulfuration are involved, which can lead to the reduction of viscosity and improvement of heavy oil quality. The results suggest that the underground heavy oil catalytic aquathermolysis is feasible in field use.