The mechanism of surface self-nanocrystallization and the evolution of grain size for TC17 were investigated by means of X-ray diffraction (XRD), micro-hardness testing and transmission electron microscope (TEM). The results show that the nanostructures are obtained by the supersonic fine particles bombarding (SFPB), and have obvious working-hardening on the surface layer of TC17 alloy. Compared with the matrix material, the surface microhardness is increased by one time. The thickness of the nanostructured layer increases with the extension of SFPB treatment time and finally is stabilized when the SFPB treatment time is more than 30 min. The size of equiaxed nano-crystallites is approximately 16.3 nm, which exhibits a random crystallographic orientation on the top surface layer. The surface self-nanocrystallization could be attributed to dislocation glide and division. Under the multidirectional loads of repeating actions, a large number of dislocations are formed. The dislocation slippage, dislocation accumulation and interaction of dislocation form the dislocation cells or walls. The dislocations continue to slip, and then form subboundary or subgrain. With increasing the strain, more and more dislocations are born and annihilated in the subboundary, so that the subboundary and subgrain transform into grain boundary and grain. When the forming and annihilating rate of the dislocations achieve the balance, the grains achieve to nanometer level.