As high-grade nickel sulfide concentrates become increasingly scarce, high-magnesium nickel sulfide concentrates will become the primary feedstock for pyrometallurgical nickel production. High magnesium content can lead to issues such as unreasonable slag composition, high slag viscosity, and high nickel losses during the smelting process; existing processes struggle to meet the smelting requirements of high-magnesium nickel sulfide concentrates. To address these issues, this study investigated the reaction mechanisms during the matte smelting of high-magnesium nickel sulfide concentrates. It examined the effects of key parameters – including nickel-matte grade, temperature, Fe/SiO₂, and CaO/SiO₂ ratios – on slag phase composition, physical properties, and nickel loss, thereby identifying optimal slag compositions and process parameters. The results indicate that under optimal process conditions (nickel matte grade of 35%, temperature of 1425°C, Fe/SiO2 ratio of 0.8, and CaO/SiO₂ ratio of 0.15), the nickel content in the slag can be reduced to 0.27 wt.%. These results meet industrial production standards, achieve efficient nickel enrichment and provide a theoretical basis and practical guidance for ensuring the smooth operation of the smelting process for high-magnesium nickel concentrates.