In this work, the role of citric acid (CA) in preparing CoMo/Al2O3 catalyst was investigated from aqueous impregnation solution to catalytic active site formation by laser Raman, UV vis, in situ IR and mass spectrometry characterizations. COS and CO were employed as probe molecules to illustrate the formation of active site during and after catalyst sulfidation, respectively. Citric acid mainly coordinates with Mo, resulting in the presence of Mo CA complex, which consequently enlarge the temperature window of MoS2 formation. Parallels between in situ IR and activity testing suggest that citric acid addition increases the concentration of CoMoS active site without significantly changing its intrinsic activity. It is concluded that the essential role of CA in preparing CoMo/Al2O3 catalyst is mainly ascribed to a tuning effect on the 2D morphology of MoS2 slabs, resulting in MoS2 exposes more S-edge to accommodate a cobalt atom to form more CoMoS sites.