Abstract: Heavy oil pyrolysis reaction was studied with formic acid as hydrogen donor and oil-soluble organic home-made nickel salt as catalyst，in high-pressure type I CWYF - reactor under simulated thermal recovery conditions. The effect of added hydrogen donor in water on the viscosity，group composition and sulfur content of heavy oil during the hydrothermal catalytic pyrolysis reaction was investigated，and the thermal conversion behavior of asphaltene before and after the catalytic pyrolysis reaction was analyzed by using TG - DTA analysis. The results showed that heavy oil viscosity reduction rate and contents of saturated hydrocarbons，aromatic hydrocarbon increased，but contents of resin，asphaltene decreased，and sulfur content decreased，with increasing mass fraction of hydrogen donor during the pyrolysis reaction. Through hydrogen donor hydrothermal catalytic pyrolysis reaction，the asphaltene TG - DTA curve analysis indicated that weight loss of asphaltene was higher after the hydrothermal catalytic pyrolysis reaction than before the reaction. After hydrothermal catalytic pyrolysis reaction of heavy oil，the stability of asphaltene became worse.