Under the simulated thermal recovery condition, fundamental experiments were carried out in the high temperature high pressure reactor to study the aquathermolysis reaction of heavy oil using formic acid as hydrogen donor and oil-soluble organic home-made nickel salt as catalytic. Urea was selected optimally from the alternative chemical addictivess of ammonium carbonate, urea and ammonium hydrocarbonate and its dosage was 20wt%. The affect of the adding addictives in water for viscosity, group composition and element content of heavy oil during the hydrogen donor catalytic pyrolysis reaction, was researched. It was revealed that there mechanism in the heavy oil viscosity reduction reaction process. The addition of addictives had not only some degree of viscosity reduction function, but also synergistic interaction to the catalyzed reaction with hydrogen donor. Compared to the oil sample from reaction without addictives, there was no obvious change on group compositions, the contents of atom C, H, S, N and O nearly kept unchanged. the introduction of addictives basically did not change structure of heavy oil.