Performance of slag glass ceramic largely depends on the phase separation structure formed in heat treatment. In the paper, the crystallization properties and its relation with phase separation structure of blast furnace slag (BFS) are researched. Three water-quenched samples and an air-quenched sample were respectively prepared. After conducting temper experiments and analyzing XRD and DTA results, the following conclusions have been acquired. Two crystallization regions exist in BFS system. In parent glass with phase separation structure, Ca3Mg(SiO4)2 and Ca2MgSi2O7 would form at lower temperature but Ca2Al2Si2O7 would appear at higher temperature than that in parent glass with homogenous structure. The former contributes to the existence of lower polymerized [SiO4] units and more Ca2+ or Mg2+ in Ca-rich phase of phase separation glass, while the later is due to the diffusing hindrance of [AlO4] in diffusing path and interface resistance in phase separation structure.