This paper studies the blast furnace slag glass phase structure by a series of analysis methods. In glass phase, both Si and Al ions are confirmed to occupy only tetrahedral sites, while the [SiO4]4- and [AlO4]5- are separated by Ca and Mg. Furthermore, the glass structure corresponds to micro-crystal model, which means it contains some nano-scaled micro-crystals in the glass phase. In addition, the slag glass may separate into two phases: a calcium rich phase and a silica rich phase. According to devitrification experiment, it has been inferred that the chemical composition and structure of silica rich phase are close to that of akermanite,which means most of Si is distribute around Mg.