Abstract: This paper present an automatic tracking system applied to vacuum electron beam welding and the process of design and experiment of this system.When the annular parts of a spaceflight apparatus were welded,the centre of the rotation of electron gun and the centre of the annular weld are usually not superposed.In this teaching process,a litter bundle of electron beam is used to scan the weld groove,the amount of the secondary electron reflected from workpiece is different when the electron beam scans the both sides and the center of weld groove.The different amount can indicate the position of weld,then the computer will record the deviation between the electron beam spot and the center of the weld groove.The computer will analyse the data and put the data into the storage softwards.During the welding process,the computer will modify the position of the electron gun with the deviation to make the electron beam spot on the annular weld groove.