Abstract: Aiming at making the study of surface nanocrystallization on 7A52 aluminum alloy welded joint comprehensive, the tandem welded joint of 7A52 aluminum alloy was subjected to an ultrasonic peening with 2A of current and 1.25 min/cm² of impact speed. The microstructure of the surface layer of the welded joint after ultrasonic peening was characterized and analyzed by scanning electron microscope(SEM), and transmission electron microscopy(TEM) observations. After ultrasonic peening, the thickness of deformed surface layer of the welded joint was about 70 μm at weld metal(WM) and heat affected zone(HAZ), about 50μm at base metal(BM). The grains of the welded surface were refined in which the grain size of WM surface reached 70-300 nm, and the grain size of BM surface reached 50-500 nm.