Amphiphilic nickel chelate catalyst was prepared and used in the viscosity reduction process of extra-heavy oil by aquathermolysis. The results showed that the amphiphilic nickel chelate had a good catalytic performance in the aquathermolysis, and the viscosity of extra-heavy oil San56-13-19 was reduced by 96.26% at 200 degrees C. The structure and group composition of the heavy oil were tested and analyzed by thin layer chromatography, Fourier transform infrared, gas chromatography, elemental analyzer, vapor pressure osmometry, and hydrogen nuclear magnetic resonance before and after the aquathermolysis. It was found that the pyrolysis of asphaltene played a key role in viscosity reduction during the catalytic aquathermolysis. Many reactions happened during the catalytic aquathermolysis, such as pyrolysis; ring opening; ring closing of C-S, C-C, C-N, C-O, and hydrogenate; and desulfurization of C=C, C=N, C=O, and S=O. It also found that aromatic sulfonic nickel causes more changes of the asphaltenes and sulfur-containing groups.