Understanding the crystallization behaviour of a blast furnace (BF) slag under a phase change cooling process and determining a suitable cooling rate are of tremendous importance for the double-wins centrifugal granulation and heat recovery technology. Moreover, the binary basicity shows a significant effect on the crystallization behaviour. In the study presented here, four synthetic BF slags with a binary basicity ranging from 1.0 to 1.4 were prepared. Moreover, the liquidus and glass transformation temperatures of the BF slags were obtained by differential scanning calorimetry (DSC). Furthermore, an improved directional solidification technique and Xray diffractometer (XRD) were combined to explore the effect of the binary basicity on the crystallization region, critical supercooling degree and cooling rate, and crystal phase type and content. The results indicate that the crystallization region narrows gradually with a decrease in the average cooling rate. Moreover, the increase in the binary basicity results in a gradual decrease in the critical supercooling degree and a linear increase in the critical cooling rate. In contrast, the primary phase transforms from akermanite to merwinite as the binary basicity increases from 1.3 to 1.4, which leads to a sharp decrease in the critical supercooling degree and a rapid increase in the critical cooling rate. In addition, the critical cooling rate of the BF slag in the cooling process with a variable cooling rate is approximately 50% of that when the cooling process is conducted with a constant cooling rate.