Abstract: In this paper,the influence of beam parameters and vacuum chamber pressure upon the shape of etectron beam has been studied.It is found that the diameter and the location of the focal spot were changed With the change of EB current.The practice of using low beam current to determine the location of beam focal spot would therefore be irrational in the case of welding heavy section metal plate with high beam current.Emphasis is also laid upon the influnce of the change of vacuum chamber pressure on the shape of EB in the 13.332Pa~133.32Pa range,for which the experimental formulas for determining the location and diameter of the beam focal spot are given.