Abstract: NiMo/Al2O3 catalyst was prepared by impregnation method using alumina support from appropriate pseudo-boehmite. The effect of Ni/(Ni+Mo) atomic ratio on the activity of NiMo/Al2O3 catalyst was investigated and optimized ratio was acquired. The activity of NiMo/Al2O3 catalyst could be further improved by adding optimum amount of phosphorus promoter and chelating agent into the impregnation solution. The characterization of said impregnation solution by Raman spectroscopy and UV-Vis spectrophotometer showed that phosphorus-molybdenum heteropolyanions of [P2Mo5O23]6- existed in the solution, besides, some interactions between chelating agent and Ni2+ cations also happened. By optimizing preparation conditions, RS-1100 hydrodesulfurization catalyst with high activity was developed. The pilot-plant test results show that the activity of RS-1100 catalyst is higher than that of commercial RS-1000 catalyst at ultra-deep hydrodesulfurization stage, whereas, the bulk density of RS-1100 catalyst is lower. Diesel oil meeting European emission standard V can be produced by RS-1100 catalyst under suitable hydrogenation reaction conditions.