In the present work, a finite difference model (FDM) is built to predict the transient temperature field of the aluminum billet in the gradient cooling process. The billet is divided in 5 segments, each of which is corresponding with one water cooling ring, and all of which are water-cooled for different times respectively in order to obtain the target temperature gradient. On the following air cooling stage, the temperature of the outer layers, especially the surface, rises first, then falls slowly. With the rising of the surface temperature, the temperature deviation from the center to the surface is narrowed intensively. In comparison with the commercial finite element method (FEM) software, Deform 3D, the FDM model gets nearly the same prediction results as FEM simulation does.