Additive manufacturing has flourished as an advanced technique to process metals and alloys. However, this strategy usually introduces undesired defects that deteriorates the mechanical performance of structural materials. Herein laser shock peening (LSP) is proposed as an efficient strengthening approach to reshape the surface morphology of a prototypical dual-phase AlCoCrFeNi high-entropy alloy (HEA) after additive manufacturing, in which remarkable strengthening is achieved. Combined electron back scatter diffraction and transmission electron microscope characterizations reveal that the mechanical enhancement is attributed to the grain refinement and accumulation of dislocations at the impact surface. In extreme condition of LSP, the grain refinement is not accommodated by the conventional dynamic recrystallization anymore, but a novel mechanism of parental columnar grain rotation which can be rationalized by a continuum-level theory from a geometrical perspective. The new mechanism is verified by large-scale atomistic simulations which further recognizes the critical role of multiple unstable dislocation slip and amorphization in formation of smaller grains under shock. Our strategy offers a promising pathway toward polishing morphology of HEAs and thus, prohibiting the potential intrinsic defected induced-mechanical degradation of the additively manufactured metals and alloys via novel microscopic mechanism.