Abstract: The disturbance mechanism of ERA is simply the interactionbetween the thin moving plate and the shaped charge jet. But this is not thecase but when the shaped charge jet impacts ERA in a normalpenetration. This paper studies the physical process of ERA and theshaped charge jet, based on X-ray photographes. It is concluded that it isthe pressure at the denotation site that causes the early jet breakup.Based onthe breakup mode of jet in the air, applying the elastic-plastic theory,the mode of jet particulate in ERA is obtained. With this mode, thepenetration of the residual jet can be evaluated. The theoretical results agree well with experiment data.