Magnesium silicates pose severe environmental issues in hydrometallurgical processes. Copper-nickel sulfide ores are the primary source for nickel production, but their increasingly complex properties make separation difficult, causing slimy magnesium silicate minerals to be entrained into the concentrate with the foam, leading to magnesium contamination. Studies have found that sodium alginate (SA), as an environmentally friendly polymer reagent, can effectively inhibit serpentine. This work investigates the effect of SA on the flotation behavior of pentlandite and serpentine in a sodium hexametaphosphate (SHMP) system. Sedimentation tests show that only a low dosage of SA can effectively settle serpentine, while excessive SA disperses serpentine. Flotation tests indicate that SA can further increase the difference in recovery rates between pentlandite and serpentine based on SHMP. Results from artificial mixed ore tests demonstrate that the recovery rate of serpentine decreases from 51.22% to 37.37% with the addition of SA, with minimal impact on the recovery rate of pentlandite. To explain the impact of the SA adsorption process on the mineral states, the following tests and analyses were conducted: TOC and QCM-D results indicate that SA mainly adsorbs onto serpentine, forming rigid adsorption at low dosages and flexible adsorption at high dosages. Zeta potential tests reveal changes in the surface potential of the minerals, explaining the transition from electrostatic attraction to electrostatic repulsion between pentlandite and serpentine. FTIR and XPS results show that SA chemically reacts with the Mg and Si sites on the serpentine surface through its -COO- groups. In summary, in the SHMP system, the use of gel adsorption and the chemical interaction of SA to coagulate and depress serpentine reduces its entrainment into the concentrate, achieving efficient separation of pentlandite and serpentine. However, high dosages of SA do not yield satisfactory results.