Nickel is a very important metal raw material, a silvery-white metal, mainly used in the production of stainless steel. Nickel resources that have been exploited in China are almost low grade copper-nickel sulfide ores with Mg-containing silicate gangue minerals. Jinchuan Nickel Mine is a copper and nickel symbiotic refractory polymetallic sulfide ore with high amount of magnesium oxide, containing many associated minerals;Jinchuan Nickel Mine is Chinas largest nickel deposit and also is one of the three oversize deposits of nickel sulfide in the world. Flotation is the common method that has been used for the suppression of Mg-bearing gangue minerals in low grade copper-nickel sulfide ores. Some extensive researches and studies about copper-nickel sulfide ores flotation have been conducted but failed to find a perfect or controllable solution so far. Production practice shows that, the Mg-containing gangue minerals in copper-nickel sulfide flotation concentrate not only seriously affect the concentrate quality, but also bring a lot of trouble on the subsequent smelting, therefore, a research on magnesium reduction flotation in copper-nickel sulfide ores with Mg-containing silicate gangue minerals is relevant.The nickel ores used in this paper were from No.2 Mine Area in Jinchuan. On the base of analyzing and summarizing a large number of domestic and international related literatures, and fully analyzing the properties of the raw ore, this article put forward the new idea for floatation to decrease magnesium by applying a combination of complexing agent-inhibitor reagent. The single factor experiments and open circuit flotation experiments of citric acid-sodium hexametaphosphate reagent system were carried out. The complexing agent was used to clean the surfaces of copper minerals, nickel minerals and gangue minerals consequently increasing their floatability difference, and an inhibitor was applied to depress Mg-bearing gangue minerals.Nickel ores from No.2 Mine Area in Jinchuan were studied by some detection means, such as X-ray diffraction analysis, chemical phase analysis, multi-element analysis and infrared spectroscopy (IR) analysis. The results showed that:the nickel grade was 1.29％, copper grade was 0.83%and magnesium oxide (MgO) grade was 29.10%in the raw ore, nickel and copper mainly exist in the form of sulfide, and the distribution rate of nickel sulfide and copper sulfide were 96.28% and 72.29%respectively, the minerals were mainly pentlandite and chalcopyrite. The gangue minerals were mainly serpentine, chlorite, talc, magnetite, pyrrhotite and pyroxene.The optimum conditions of rougher flotation of citric acid-sodium hexametaphosphate system reagent were: grinding fineness of -0.074mm accounted for 90%, pH value of 8.5, citric acid used as complexing agent at dosage 1500g/t, sodium hexametaphosphate used as inhibitor reagent at dosage 400g/t, butyl xanthate used as collector at dosage 50g/t, J-622 used as frother at dosage 55g/t, rougher flotation time as 4min.Closed circuit flotation, of the new reagent regime flotation system, of one roughing, two cleanings, three scavengings, gave the final copper-nickel mixed concentrate with nickel (Ni) grade of 9.37%, copper (Cu) grade of 5.08%, nickel (Ni) recovery rate of 84.22%, copper (Cu) recovery rate of 69.93%and magnesium oxide (MgO) content of 6.39%.Nickel (Ni), copper (Cu) and magnesium oxide (MgO) contents obtained from the new reagent regime flotation system fulfilled the expected research requirements compared to contents obtained from Jinchuan existing reagent regime flotation system.And by using infrared spectroscopy (IR) analysis, the effect mechanism of reagents in copper-nickel sulfide ores flotation had been investigated. The results of FTIR showed that, new absorption peaks of reagents were found in serpentine infrared spectra after Cu2+activation, which indicated that butyl xanthate adsorbed on the surface of serpentine and citric acid had the function of cleaning the Cu2+activation on the surface of serpentine, and that after Cu2+ activation, in pentlandite infrared spectra appeared new absorption peaks proof that physical adsorption and chemical adsorption occurred between butyl xanthate and the surface of pentlandite, and also that pentlandite has a very good flotation activity.