Molten slag is broken up by supersonic air into droplets through the air quenching dry slag granulation technique. The breakup process of blast furnace slag directly determines the droplet diameter and the waste heat recovery. In order to gain deep insight into the granulation mechanism and visualize the breakup process, three-dimensional unsteady numerical simulation based on the k-ωbased shear stress transport turbulence model was conducted to simulate the transient breakup process of molten slag (k is the turbulent kinetic energy, andωis the specific dissipation rate). The coupled level-set and volume-of-fluid method was utilized to capture the sharp air–liquid interface. The results show that a flat film is formed firstly under the effects of air impingement, recirculation zone and pressure gradients. Then, the axial wave and the spanwise wave appear simultaneously and the film is broken up into ligaments owing to the generation of vortex and hole structure at the intersection of axial trough and spanwise trough. Finally, the ligaments are broken up into droplets owing to Rayleigh–Taylor instability at the air–liquid interface. The droplets smaller than 3.00 mm account for 80％, with the average diameter of 1.95 mm.