Abstract: Based on the visual image sensing method,extraction of the characteristic parameters of the weld penetration in electron beam welding of aluminum alloy was investigated with the flat plate and the slope plate experiments.The results show that the width of the feature zone(W) responds to the transition of the penetration status from the partial penetration to the full penetration during electron beam welding of aluminum alloy,but the rear part of the length of the feature zone(RL) does not.So the width of the feature zone can be used as the characteristic parameter of the penetration to monitor and control the variation of the penetration status,which provides an effective way for penetration control in electron beam welding of aluminum alloy.