<p>To improve the smelting conditions for nickel metal production in flash furnaces, the composition of the nickel slag system with a fixed Fe/SiO₂ of 1, 8.59 wt pct MgO, 1.93 wt pct Al₂O₃, and 1.61 wt pct CaO was adjusted by adding B₂O₃ within the range of 0 to 4 wt pct. Molecular dynamics simulations (at smelting temperature of 1400 °C) and experimental characterizations were employed to analyze the influence mechanism of B₂O₃ on the phase composition, microstructure, and physicochemical properties of the nickel slag, and to determine the suitable addition amount of B₂O₃. The results showed that B₂O₃ promoted the formation of pyroxene phase, reduced olivine phase, and generated a small amount of borate phase in the nickel slag. The addition of B₂O₃ complicated the nickel slag structure, which was attributed to two aspects: on one hand, B promoted the polymerization of the silicate network; on the other hand, the simple tetrahedral and triangular structures of Si and B combined to form Si_IV–B_III linkages. When the B₂O₃ content was 2 wt pct, the melting temperature, electrical conductivity, surface tension, and viscosity of the nickel slag were 1278 °C, 57.53 S·m⁻¹, 0.173 Pa s, and 0.264 N· m⁻¹, respectively. Compared with B₂O₃-free nickel slag, the melting temperature of the slag with B₂O₃ addition is reduced by 69 °C. Although its viscosity is slightly increased, it remains far below the critical smelting threshold of 0.5 Pa s. Meanwhile, the electrical conductivity and surface tension of the modified slag fall within the range of practical production requirements, fully satisfying the performance criteria for slag in the existing flash smelting process.</p>