The effects of laser shock processing (LSP) on grain size, element content and microstructure are investigated by scanning electron microscope (SEM) and transmission electron microscope (TEM) observations in the top surface, the sever plastic deformation layer, the minor plastic deformation layer and the substrate after a single laser shock processing. The influences of LSP on micro-structure and grain size of LY2 aluminum alloy are discussed, and the enhancement mechanism of a single LSP on LY2 aluminum alloy is also addressed. After a single LSP impact, the change of dislocation structure can be also clearly seen at different layers, i.e., it varies from dislocation lines (DLs) to dislocation tangles (DTs) and dense dislocation walls (DDWs), to subgrains or refined grains as functions of the distance from the top surface. It is found that the microstructure is obviously refined due to the ultra-high plastic strain induced by multiple LSP impacts.