Abstract: The lap amount of bead is critical not only for the trouble-free operation of three-dimensional welding,but also for the forming accuracy.So the interval bead lap model of the GMAW welding is established on the basis of the physical characteristics of droplet transition.Furthermore,the theoretical bead distance of flat overlaying welding layer is calculated.Actual welding test is employed for the model verification and the height of surfacing layer also is calculated.The test results agree well with the theoretical ones,which establish the foundation for the welding automation.