Abstract: In order to study the welding temperature field induce by electro slag welding (ESW) of the box column plate and diaphragm of high-rising buildings, a computational approach was developed to predict welding temperature field based on MSC.Marc software. In the proposed approach, a new heat source model named half ellipsoid volumetric heat source with uniform density was developed to simulate ESW heat input. The "birth and death" element technique was used to simulate the formation of weld bead, and temperature-dependent thermo-physical properties of SM490A tested by experiment were considered in the thermal analysis. In addition, the thermal cycles at several typical positions were measured by K-type thermo-couples. The results show that the thermal cycle predicted by numerical simulation are in good agreement with that by experiments and the shape of fusion zone predicted by FE model is much similar to that obtained by the experiment. The comparison between FEM and experiment shows that the proposed approach is an effective means to predict welding temperature field for ESW joint.