Abstract: Laser shock peening (LSP) technology was used to strengthen the surface of TC4 duplex titanium alloy (impact 0-3 times). The grain refinement mechanism of β phase and the changes of residual stress, surface hardness and fatigue strength before and after strengthening were studied. The results show that during the process of LSP, dislocations in the β phase grains firstly formed dislocation lines by slipping. As the plastic deformation intensified, dislocations piled up continuously to form dislocation walls and dislocation cells. After further movement of dislocations, sub grain boundaries were formed, and the grain refinement was achieved through dynamic recrystallization. After three times of impact strengthening, the grain size of the alloy was reduced from 21.33 μm of the original sheet to 4.68 μm. As the time of LSP increased, the residual compressive stress, microhardness and fatigue strength of the alloy surface increased.