Abstract: Four nano-Fe₃O₄ catalysts with different particle sizes were prepared by high temperature pyrolysis and low temperature two-phase reflux method; their performance in the hydrothermal cracking and viscosity reduction of Liaohe oil-field heavy oil was investigated. The results indicate that the addition of sodium sulfonate (HABS) surfactant in the preparation process can effectively improve the dispersion of Fe₃O₄ catalyst in heavy oil for hydrothermal cracking; the HABS-modified 9 nm Fe₃O₄ catalyst prepared by high temperature pyrolysis exhibits the best viscosity reduction performance. In a reaction system with 250 g heavy oil and 0.75 g n-hexane as hydrogen donor, where the mass ratio of heavy oil:catalyst:reservoir water is 100:0.3:30, the viscosity of heavy oil can be reduced from 86200 to 2065 mPa·s after reaction at 240℃ for 24 h; the viscosity reduction rate is as high as 97.6%. Analysis on the reaction mechanism suggests that the nano-Fe₃O₄ catalyst attacks the C-S bond on the long-chain heavy oil, breaks the bond and converts the heavy component into light component.