Abstract: In this paper,by the use of the formula for 3-dimensional temperature field of welding on unlimited plate of limited thickness and by the use of the method of least square,the authors get a best approach for the calculated thermal cycle curve to the actual one.A new "test and calculate" method is deduced for the determination of the concentration factor K for heat source of welding.It is simple and reliable,and involves no severe condition for experiment.A curve showing the relation between K and welding current under given specifications is established and supplemented with analysis.