The mechanism and the effect of microstructure evolution on the surface microhardness of TC6 titanium alloys after laser peening (LP) are studied. The results show that both of the surface microhardness and the residual stress of materials after LP increase with the increase of the power density, and the depths of the microhardness effect layer and the residual stress effect layer caused by LP also increase with the increase of the power density, and the depths are 500-600 μm. The effect mechanism of LP on the microhardness of TC6 titanium alloys consists of the fine grain strengthening and the dislocation strengthening. The number of large and small angle grain boundaries increases because of the grain refinement and the grain boundary can prevent the materials to yield. High density dislocations produced by LP can improve the yield strength of materials, hinder the movement of dislocations and restrain the crack initiation, which can improve the mechanical property of materials.