Abstract: A series of CoMo/Al2O3 catalysts with different metal ratio（Co/(Co+Mo)） were prepared using citric acid (CA) as a chelating agent by stepwise impregnation method. The effect of CA on the role of promoter was studied and the prepared CoMo/Al2O3 catalysts were characterized by BET，XRD，XPS and HRTEM techniques. The results show that CA can increase the dispersion of Co and Mo, and enhance the promotion role of the promoter，especially at higher metal ratio. The activity of 4, 6-dimethyldibenzothiophene hydrodesulfurization show a trend of increase first than decrease as increasing the metal ratio, and reaches its highest value at the ratio around 0.3. Meanwhile, it is apparent that the catalytic activity obtains an important enhancement when the catalysts are prepared in presence of CA，especially at higher metal ratio.