In this paper, the causes of the incompletely filled groove of the butt welds in the fully mechanized welded underframe of the standard EMU at a speed of 350 km per hour were analyzed from the aspects of design structure and technological methods. In the aspects of design structure,intermittent welding technology,welding sequence,welding process parameters and so on,an optimization program has been developed and implemented in the project to completely solve the problem of incompletely filled groove during fully mechanized welding. As a result,weld repair amount dropped observably,which greatly improved the flatness after welding improved greatly,effectively reduce the work amount,suitable for mass production of plug-in aluminum alloy underframe to be fully mechanized welded.