Abstract: It has been found that when the Gaussian or Double-elliptic mode of heat flux distribution was employed in numerical simulation of GTAW(gas tungsten arc welding) welding pool behaviors there was marked difference between the predicted and practical pool geometry at the rear of weld pool.The reason why such difference exists was analyzed,and three principles guiding how to determine the calculating zone of heat source on the work piece were put forward in designing the numerical simulation program.Through appropriate selecting the calculating zone radius of welding arc on the work piece and making it matched with the distribution parameters of Double-elliptic mode of heat flux,the trailing problem of weld pool rear in previous models had been successfully solved,and the numerical simulation precision had been greatly improved for GTAW weld pool geometry.