Abstract: The finite element analysis model for the arc welding process of the 800 MPa ultra-fine grain steel was established using ANSYS, a finite element analysis software. And the welding thermal cycles under different welding parameters were simulated by this model. Then the actual welding test as verification was carried out subsequently. Results show that temperature distribution from simulation was in accordance with that from actual test at the same welding parameters, which prove the exactness of FEM. When the welding heat input was over 845 J·mm-1 the cooling time increased significantly, resulting in the microstructure of the welding heat affected zone coarsening seriously.