Blast furnace (BF) slag is the principal byproduct formed during the iron-making process, which generates a considerable amount of thermal energy. However, to simultaneously recover the maximum amount of residual heat and obtain vitreous slag using a low conduction coefficient of air is a primary challenge of the dry granulation technique, which has not been solved by either fundamental research or engineering applications. In the current study, the evolution of temperature inside molten BF slag was experimentally investigated using the directional solidification technique to calculate the local heat-transfer coefficient and the average cooling rates. Furthermore, the effects of the average cooling rate on the vitreous phase content of the BF slag during solidification were investigated based on X-ray diffraction (XRD) technology. The results indicate that the average cooling rate from the cooling side to the adiabatic side gradually decreased due to the increase in heat resistance and latent heat released. Moreover, the vitreous phase content of the BF slag at different positions decreased as the average cooling rate decreased. Conversely, the crystal phase content increased gradually and released more latent heat, which further reduced the average cooling rate. In addition, the crystal phase obtained at different positions was presented as akermanite, and the average critical cooling rate was 10.6 degrees C s(-1). Lastly, dimensionless correlations were developed to predict the distributions of the average cooling rate and the vitreous phase content of the BF slag along the vertical direction, and the theoretical value has a good agreement with the experimental data.