Abstract: An image sensing system without auxiliary light was es-tablished according to the characteristic of electron beam welding.Images during the electron beam welding of an aluminum alloy were acquired.With the image-processing algorithm designed,the original characteristic parameters of the feature zone defined previously were extracted.Then,the distribution curves of the characteristic parameters the front half of the length(FL) and the width(W) were obtained by low-pass filtering.It showed that the variation of FL was the same as that of W by calculating the ratio of FL to W and carrying out the correlation analysis was of FL and W.Either FL or W was alternatively determined to describe the front half of the feature zone.