Abstract: To know the actual welding heat input is the prerequisite for solving in a precise manner the thermal elasto-plastic problems and for evaluating residual stresses and distortions when more accurate results are expected.For this reason,the determination of the heat source efficiency η is a pressing subject in the course of developing welding mechanics.In this paper,the authors present a "measurng-calculating method" which provides a new and accurate shortcut to determine the heat source efficiency η.By introducing the proper melting latent heat coefficient p the actual efficiency η_f for linear heat source was given in a modified formula.The experimental data on four different thin plates by using TIG welding agreed well with the theoretical resuets of calculation.It is proved by theoretical deduction and confirmed by experiment that the dimensionless value of linear heat input for forming weld bead is the reciprocal of coefficient η_f.The principle for selecting welding parameters given in this paper could also be used for other fusion welding heat sources,if their conditions are in compliance with those of the linear heat source.The coefficient η is not only related to the welding technology and parameters but also varies with the thermophysiocal characteristics of the materials to be welded.