2A12 aluminum alloy welded joint was processed through high frequency impacting and rolling (HFIR). The microstructure features and properties of the plastic deformation layer were observed by SEM, XRD, TEM, EBSD and micro-hardness test. For different deformation layer of base metal and weld center, the effects of T precipitated phases and S precipitated phases on grain refinement process were studied in detail. Results indicated that work hardening and grain refinement were achieved in a certain depth from the top surface of welded joint. In the base metal, with the depth from the treated surface of base metal decreasing, the motion of dislocation was blocked by T precipitated phase and dislocation lines gradually intersected. The T precipitated phases were the intersection of sub-grain boundaries. While, in the weld center, with the depth from the treated surface decreasing, S precipitated phases were rotated and sheared by lots of dislocations, and broke down to the nanometer scale.