Nano-nickel catalyst was prepared in microemulsion system, and used in the viscosity reduction process of extra-heavy oil by aquathermolysis. The results showed that the nano-nickel had a good catalytic performance in the aquathermolysis, and the viscosity of extra-heavy oil San56-13-19 was reduced by 90.36% at 200 ı C. The structure and group composition of the heavy oil were tested and analyzed by thin-layer chromatography-flame ionization detection, Fourier transform infrared spectroscopy, elemental analysis, vapor phase osmometer, and hydrogen-1 nuclear magnetic resonance before and after the aquathermolysis. It was found that the pyrolysis of asphaltene played a key role in viscosity reduction, and the O-containing products such as alcohol, carboxylate, and fatty acid can reduce the viscosity of heavy oil during the catalytic aquathermolysis.