Abstract: High temperature aquathermolysis is generally carried out between heavy oil and hot water with high pressure,but the aquathermolysis reaction between over-heated steam and heavy oil is hardly studied.In order to conduct this research,based on investigation,a high temperature and high pressure autoclave whose volume is changeable is designed to substitute for the autoclave whose volume is fixed,so the aquathermolysis of heavy oil in over-heated steam is achieved.By means of the equipment,the influence of over-heated steam on heavy oil aquathermolysis under the condition of different temperature and different water content is studied.Result shows there is an apparent aquathermolysis reaction between crude oil and over-heated steam.Further analysis demonstrates after aquathermolysis reaction,oil viscosity reduces the amount of saturated and aromatic hydrocarbons increases,resin and asphaltenes decrease,carbon atom content in crude oil decreases，hydrogen atom content increases greatly and sulfur atom content decreases.As water content increases the crude oil viscosity decreases firstly,then increases after reaction.When water content is 30%,the viscosity is the lowest.

Key words: heavy oil, over-heated steam, viscosity reducing, aquathermolysis, experiment