Abstract: Flooding phased array ultrasonic longitudinal wave technology and contact ultrasonic longitudinal wave technology are applied to non-destructive testing of Al-4.5 wt% Mg aluminum alloy rolled plates to assess equivalent dimensions and positions of discontinuous defect. The discontinuity was analyzed by using optical microscopy, SEM and EDS methods. Under the condition of considering the influence of Mg element content, the influence of chemical composition on the alloy was studied by using CALPHAD technology, the Al₇（Cr, Mn,Ti） phase proportion inferred from ultrasonic C scanning data using water-immersed phased array was verified by metallographic statistics. The results show that the coarse metal compounds are mainly composed of Al₇Cr phase, with the solid solution of Mn, Fe, Ti elements. The driving force of coarse metal compounds is significantly increased in a specific chemical composition range, and there are relatively safe chemical composition regions.