A new phase field model coupled with external field was used to simulate 2D and 3D grain growth processing. The simulation results showed that 2D grain size distribution under external field was inhomogeneous. The evolutions of 2D grain under different external field strength were roughly the same, but the grain number was obviously different with the changes of external fields. Under the external field, the relationship between 2D grain area and time was linear. 3D grain distribution was inhomogeneous in the direction of the external field gradient. The nucleation gradually decreased in the direction of change.