Abstract: The models of both heat source and welding temperature field were developed according to the characteristic of keyhole plasma arc welding (K-PAW) process. The result of numerical simulation indicated that neither general double ellipsoidal heat source nor three-dimensional conical heat source could precisely describe the thermal process in K-PAW. A modified model of three-dimensional conical heat source was put forward. It could be used to calculate the weld width on both the top and the bottom surfaces of the workpiece exactly. But for the fusion line, the calculating precision was lower. Based on the theoretical analysis of keyhole and weld pool behaviors, a novel heat source model was employed which considered the combined action of both heat and force from the plasma arc. Finite-element analysis of the temperature fields in K-PAW was conducted, and the calculated geometry result of K-PAW weld geometry was in agreement with experimental result.