Abstract: Based on visual image sensing method,variation of the rear part of the length(L_R) and the width(W) of the feature zone in electron beam welding of aluminum alloy were investigated.The characteristic parameters describing the appearance of the aluminum alloy weld in electron beam welding were determined by analyzing the relationship between the appearance of weld and the fluctuation of RL and W.The results show that the appearance of weld can be described successfully by the variation coefficient of L_R(ν_RL) and the variation coefficient of W(ν_W) as the characteristic parameters,because the fluctuation of RL and W are the dependent variable and the independent variable of the variation of the welding pool respectively.The effect that the appearance of weld is described by both the characteristic parameters is the same.