Improvement of the selectivity of hydrodesulfurization (HDS) for hydrogenation (HYD) of olefins is crucial to produce sulfur-free (S＜0.001%) gasoline from fluid catalytic-cracked (FCC) gasoline. A series of sulfided CoMo/Al2O3 catalysts with different metal loading were prepared by pore-filling impregnation. MoS2 and CoMoS active phases on the surface of sulfided CoMo/Al2O3 catalyst were identified and analyzed quantitatively by XPS and in-situ FTIR of adsorbed CO. The results reveal that the increase in CoMoS phase on the catalyst surface improves the HDS activity and selectivity. And the HDS selectivity correlates linearly with the ratio of active site number of CoMoS and MoS2, the higher the ratio of active site number of CoMoS and MoS2, the better the HDS selectivity. In situ variable temperature FTIR analysis shows that CoMoS phase has stronger electron accepting ability than MoS2. The strong electron deficient property of CoMoS active sites is the main reason for its excellent HDS activity and selectivity.