Abstract: A 3-dimensional transient numerical simulation model for gas tungsten arc(GTA) welding 0Cr18Ni9 stainless steel sheet was developed based on SYSWELD software.A double ellipsoid heat source mode was adopted to depict the distribution of a moving GTA welding arc.The method modified the parameters of heat source was put forward.The thermo-physical properties and latent heat were taken into consideration.The transient variations of temperature fields and molten pool geometry were predicted and verified.It has been found that the calculated shape of molten pool was in agreement with the experimental one.