Abstract: The gradient nanostructures are prepared on the surface of 300M steel by laser shock processing (LSP). The microstructure and mechanical properties of 300M steels subjected to LSP with different pulse energies are characterized by three-dimensional surface profilometer, scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffractometer (XRD), nano-indentation instrument and tensile tester. The results show that the gradient nanostructures are formed on the surface of 300M steel after LSP treatment, that the grain size of the surface layer is refined from 15 nm (3 J) to about 10 nm (7 J) with the increase of the pulse energy, and that the grains turn to be amorphous. Meanwhile, a large number of crystal substructure defects such as dislocation tangles and mechanical twins are formed in the subsurface structure. With the increase of pulse energy, the dislocation density increases sharply and the number of mechanical twins increases. The nano-indentation hardness of 300M steel is significantly improved after LSP and increases with increasing pulse energy. The corresponding strength and plasticity are improved to some extent and the fracture morphology changes from typical ductile fracture to ductile-brittle mixed fracture.