After gas quenching and granulated, the molten steel slag changed into droplets. The cooling and solidification process of the droplets have been analyzed and calculated in this paper. It was suggested that there were three phases such as liquid cooling phase, nucleation and recalescence phase, and solid cooling phase. During the cooling process, there were two heat transfer modes such as convection and radiation from droplets surface to environment. On the other hand, there was a great amount of latent heat released during nucleation and solidification process. The physical model of droplets cooling has been established on the basis of heat transfer theory. And then the mathematic model with nonlinear ordinary differential equations could be extracted, which could be solved through fourth-order Runge-Kutta method. The declining law of droplets temperature along with the cooling time could be drawn by Matlab software. It takes 249ms, 83ms and 185ms for liquid cooling, nucleation and recalescence and solid cooling phase, respectively. In conclusion, it takes about 332ms to solidify entirely for the droplets, and 517ms to be cooled from 1723 to 1073K.