Abstract: The crystallization capability at high temperature and devitrification properties of glassy films in annealing of routine and optimized high-speed casting mold flux were studied. The results show that the optimized mold flux with high basicity has a higher crystallization temperature and a lower devitrified temperature;also, its crystal-forming abilitiy is much stronger in the same conditions. The main crystal phase of the optimized mold flux is Cuspidine(3CaO·2SiO₂·CaF₂) and it has larger crystalline particles. Moreover, there are a great number of gaps in the solidification structure, which can increase the roughness on the surface of flux films. Consequently, it can enhance the heat resistance of flux films and will be more suitable for high-speed casting.