Abstract: Vacuum residuum is mixed with FCC slurry oil and the mechanism of residuum modification FCC slurry oil is studied.In the course of mixed residuum with FCC slurry oil,there are two competitive processes of solution and adsorption.The solution process means that some components with similar structure and solubility parameter from FCC slurry oil and residuum will dissolve in each other and small molecules will be freed from the surface of big molecules;The adsorption process means that asphaltene adsorbs molecules with polarity similar to and some molecules with similar surface structure adsorb on each other with small particles aggregating into big ones.The process of solution is balance with process of adsorption.When the ratio of FCC slurry oil is lower;while the process of adsorption predominates,when the ratio of FCC slurry oil is higher.The combination of the two competitive processes changes the composition and compatibility of residuum colloidal system,which leads to the decrease of solvent power and stability of residuum.The properties of mixed residuum that change with temperature should be further studied.