Potential function of Fe_xO-SiO₂-CaO-MgO-"NiO" nickel slag in a two-atom model is constructed with nonempirical parameters. Microstructure and physicochemical properties of nickel slag are explored with molecular dynamics simulation. It shows that the potential energy of Fe_xO-SiO₂-CaO-MgO-"NiO" nickel slag can be characterized well with BMH(Born-Mayer-Huggins) potential function. It is beneficial to diffusion as CaO content of nickel slag is 15 wt.%, at which the coordination number of Si⁴⁺-Si⁴⁺ and the polymerization degree of slag are the lowest. The binding capacity of Fe with O^2- become stronger as Fe element is converted from Fe²⁺ to Fe³⁺, which makes the slag difficult to diffuse. It makes the viscosity of nickel slag increases rapidly and the smelting conditions become deteriorate. Therefore, the ratio of Fe²⁺/Fe³⁺ should be controlled strictly during nickel flash smelting. The simulated viscosity is consistent with measured value. It shows that the potential function reflects physical and chemical properties of nickel slag well.