To investigate the problems of high loss of nickel metal in slag for nickel flash smelting, the mechanism of Fe/SiO2 ratio(mass ratio of Fe element to silica, hereinafter referred to as Fe/SiO2 ratio) on the separation characteristics of matte and slag was analyzed based on the bridge oxygen structure, FeO activity and valence change law of Fe in molten slag. The results show that the activity of FeO and the contents of Fe2+ and Fe3 + in nickel slag increase with increasing the addition of Fe/SiO2 ratio from 1.1 to 1.5. The increased Fe2+, Fe3+ and O2- promote the dissociation of complex silicate ion groups. The contents of Q1 and Q2 in silicate are reduced from 32.54% and 50.25% to 23.84% and 38.81%, respectively, while the content of Q0 is increased from 17.21% to 37.33%. The main structure of nickel slag is gradually transformed from a complex silicate structure to a Fe3+ connected SiO44- tetrahedron structure. The mass distribution ratio of Ni in matte and slag reaches the maximum value when m(Fe)/m(SiO2) = 1.2, which is greater than that of the original smelting slag under the same condition.