The aquathermolysis (AqTh) of Liaohe heavy crude oil (HCO) in a 300ml autoclave is investigated with an oil soluble catalyst and formic acid as hydrogen donor at 280°C for 24h in N2 atmosphere，with initial pressure 8.1 MPa. The catalyst is a green viscous liquid of nD=1.4737 and made from naphthenic acid and nickel sulfate. The viscosity of HCO is temperature sensitive, being of 3716 mPa·s at 50°C and shear rate 44.1 s-1.The plain catalytic AqTh of HCO leads to a percentage viscosity reduction(PVR) of 64.69%，to an increase in saturates and aromatics contents from 24.32% and 36.89% to 26.12% and 38.08%, and to a decrease in resins, asphaltenes, and sulfur contents from 30.27%，8.52%, and 0.5650% to 28.27%,7.53%,and 0.3365%, respectively; when formic acid is introduced at dosage of 1~7% HCO mass, the PVR is continuously raised to 69.16~87.02%, the saturates and aromatics contents increased to 27.73~31.12% and 39.68~41.26% and the resins, asphaltenes, and sulfur contents decreased to 26.29~24.12%, 6.66~3.50% and 0.3095~0.0742%,respectively. The IR-spectrometry shows that decarboxylation is conducted and number of aromatic rings is lessened in hydrogen donor catalytic AqTh. The role of formic acid and the functioning mechanisms involved inatalytic AqTh of HCO are discussed.