Abstract: In order to clarify the influence of forced cooling on the width of heat affected zone(HAZ) and Δt_8/5 of electro slag welding(ESW),a computational approach based on MSC.Marc software was developed to predict the welding temperature field. In the proposed approach, welding temperature field induced by ESW under varying cooling conditions (air, copper and water cooling) were computed by using a half ellipsoid volumetric heat source model with uniform density and the temperature-dependent thermo-physical properties of SM490A was obtained from experiment. In addition, the thermal cycles at several typical locations were measured in the ESW joint by K-type thermo-couple. The results show that the temperature histories predicted by numerical simulation are in good agreement with experimental results. The numerical results indicate that forced cooling has a limited influence on the size of fusion zone, however, copper cooling and water cooling can significantly reduce the width of HAZ and largely shorten Δt_8/5 time.