Abstract: The catalyst with molybdate as the precursor for the slurry-bed hydroprocessing of residue needs to be sulfurized for higher catalytic activity. In this study, ammonium molybdate is dispersed in lubricant oil and sulfurized under different conditions. XPS is employed to analyze the surface composition of the sulfurized products. Sodium sulfide is employed as sulfurization reagent and ammonium chloride is employed as auxiliary sulfurization reagent. The sulfurization conditions examined in this study included the amount of NH4Cl added and whether the sulfurization product is heated under hydrogen or not. Chemical probe method is employed to measure the hydrogenation activity of the sulfurized catalysts. Anthracene is employed as the probe to undergo the hydrogenation with the existence of sulfurized catalysts, and the anthracene conversion is used to show the hydrogenation activity of the catalyst used. According to the XPS spectra obtained, the molar ratio of molybdenum of different valences and the molar ratio of sulfur to molybdenum is considered respectively. It is discovered that under different sulfurization conditions, the molar proportion of MoIV and the molar ratio of sulfur to molybdenum do not form a direct ratio, thus it is concluded that the sulfides of molybdenum in the sulfurization products are not only composed by MoS2, so that a new definition of sulfurization ratio is suggested. When the sulfurization product is not heated under hydrogen, the molar fraction of MoIV and the sulfurization ratio is much lower than that of being heated. It is also observed that the ratio of MoIV and the sulfurization ration increase with the increase of the amount of the NH4Cl, which results from the special sulfurization mechanism of the molybdate. The catalytic activity measured using the method of chemical probe shows that the catalytic activity is promoted along with the molar ratio of ammonium chloride added, which is in accord with the results obtained by XPS.