Abstract: This paper briefly introduced ULSD (ultra low sulfur diesel) combined catalyst system and process developed by Fushun Research Institute of Petroleum and Petrochemicals. Test results of pilot plant showed that compared with applying single catalyst，using this combined catalyst system, under the same process conditions, clean diesel oil with much lower sulfur content could be acquired. Commercial cases of this technology, a 3.3 Mt/a diesel hydrotreating unit in Shanghai Petrochemical Company and a 3.0 Mt/a diesel hydrotreating unit in Zhenhai Petrochemical Company, indicated that FHUDS-2/FHUDS-5 combined catalyst system exhibited excellent deep hydrodesulfurization performance, which could provide diesel oil with sulfur content meeting the requirement of GB Ⅳ emission standard under mild process conditions.