A nano-crystal layer of weld joints surface was generated after the UIT (Ultrasonic impact treatment) and A-UIT (Aging and ultrasonic impact compound treatment) treating 7A52 aluminum alloy welded joints. Results indicate that the grain size of weld joints is finer and the surface hardness of the weld joints increases more significantly after A-UIT. Compared with the UIT, which the surface hardening mechanism of 7A52 aluminum alloy welded joints is fine grained strengthening, the surface hardening of 7A52 aluminum alloy welded joints obviously improved with the common effect of fine strengthening and precipitated phase strengthening after the A-UIT.