Abstract: MgO content in the nickel-copper mixed concentrate is too high since Cu²⁺ and Ni²⁺ ions of Jinchuan nickel ore can strongly activate a large amount of Mg-containing silicate gangue minerals，and the grade of Ni and Cu is difficult to be improved.To achieve the goal of decreasing magnesium and improving quality，the dressing test was carried out under the reagent system of citric acid-modified starch combination.The results showed that，at the grinding fineness of 90.12% -0.074 mm，through the closed-circuit process of one roughing-two cleaning-three scavenging，and middlings returning in proper order into the flow，the final copper-nickel mixed concentrate with Ni grade of 9.03%，Cu grade of 5.18%，MgO content of 6.18%，nickel recovery of 85.30%，copper reovery of 72.82% was obtained.Compared with the laboratory test index of simulating in-situ process， concentrate grade of Ni，Cu is increased by 0.28 and 0.07 percentage points respectively，concentrate recovery rate of Ni，Cu is increased by 3.41 and 1.04 percentage points respectively，and content of MgO drops 0.58 percentage points.Therefore，citric acid-modified starch can significantly decrease magnesium content and improve the quality of nickel-copper mixed concentrate in copper-nickel sulfide ore，which is rich in magnesium silicate gangue minerals，flotation process.

Key words: Citric acid, Modified starch, Complexing agent, Inhibitors, Copper-nickel sulfide ore, Mg-bearing gangue