Abstract: Twelve times of the thermal simulation experiment were done with samples from viscous oil pools in the Xinglongtai oil reservoirs of Du-84 block,Shuguang-1 district,Liaohe oilfield.The results of the experiment indicated that the essential factor resulting in formation damage was asphaltene precipitation.The phenomenon that asphaltene precipitated in pores was very prevalent.The aggregated asphaltene took the shapes of star spots,films,patches or even networks.The damage degree of asphaltene precipitation to porosity and permeability increased with temperature.It increased with pressure when the pressure was less than 11.5MPa,but decreased with pressure when the pressure was more than 11.5MPa.The average damage rate of permeability was above 80%.It belonged to strong damage.The damage rate of porosity was only 1.3%-6.5%.The major affecting factors of asphaltene precipitation were the component of viscous oil,temperature,pressure and solvent.