The ZK60 magnesium alloy sheet butt-welded in gas tungsten arc welding with different heat input are analyzed and simulated. A double-ellipsoid heat source and the temperature-dependent thermo-physical properties of ZK60 magnesium alloy sheet are employed for performing a non-linear transient thermal analysis by a finite element method. The effect of heat input on welding seam geometry has been studied. As a calculated result, the welding voltage, welding speed and the welding current are the main factors to determine the welding seam geometry. The different welding temperature fields between the general gas tungsten arc welding (GTAW) and GTAW hybrid a longitudinal electromagnetic field (LMF-GTAW) have been achieved.