Abstract: Armed at the large areas lamination defect in the ellipsoidal heads, its reliability is analyzed without stress concentration. According to the feature of pressure vessel working medium and loading method, the area monitoring of lamination defect is done by acoustic emission testing technique. In order to reduce the useless signal caused by the change loading during the experiment, the guard sensors technology is adopted. The results indicate that there exists the probability of the lamination defect can spread under the alternating load.