Abstract: The FCC gasolines in three domestic refineries were cut into narrow fractions to analyze the sulfur and olefin content of each fraction. The aim is to see the possibility if the fractions < 105 ℃ with more olefins can be put into LCN as much as possible on the premise of ensuring total sulfur of LCN≯50 μg/g and reducing the octane number loss due to olefins saturation in the process of HCN hydrodesulphurization. The octane number loss of the FCC gasoline after hydrodesulphurization process with or without pre-hydrogenation was compared. The results indicate that the cutting point of HCN can be greatly increased by process with pre-hydrogenation and the product gasoline is in compliance with national phase IV with less octane number loss.