Abstract: Based on the composition of coke in FCC reaction system，the existence of unvaporized oil was determined. The generation and concentration process of unvaporized oil was discussed, and the generation conditions and the hazards of unvaporized oil were analyzed. Six FCCUs were selected to judge the degree of feedstock vaporization by comparing the temperature of reaction vaporization sector and the dew point of feedstock，and the causes of the generation of unvaporized oil were analyzed from the microscopic point of view. Finally，the control measures were proposed to guide the production. It was found that it was difficult to avoid the occurrence of unvaporized oil in RFCC unit，though it had little impact on the operation of most units. It would cause coking of reaction system and scaling of flue gas expander under specific conditions. Therefore，the generation of unvaporized oil should be reduced as much as possible.

Key words: fluid catalytic cracking, unvaporized feedstock, settler coking, flue gas turbine scaling