Abstract: The physical and mathematical models of solidification process of spherical molten steel slag granule have been established in this paper.The temperature fields of slag sphere and its outer gas could be calculated through the FLUENT software.Consequently, the solidification time of the slag sphere with different diameter could be gained.It is believed that the numerical simulated results would provide the theoretical basis for granulating molten slag and heat recovery equipment layout of molten slag.The numerical simulation results showed that liquid slag sphere could release more than 80% of the phase-change heat in 2 s when the initial condition as follows:the temperature of molten slag being 1 823 K, the diameter of granule being 1~3 mm, the temperature of cooling gas being 373 K, the velocity of gas flow being 1~20 m/s.It is concluded that there could not be conglutination between slag spheres after a certain of cooling time if the molten steel slag has been granulated to 1~3 mm spheres.