Pure magnesium is a very promising material in the fields of biomedical and engineering. Obtaining pure magnesium with superior mechanical properties has consistently been a significant challenge in the area of materials science. This study focuses on investigating the processing method and strengthening mechanism of pure magnesium by ultra-precision cutting. The research results show that the pure magnesium grains were significantly refined after ultra-precision cutting. The average grain size reduced from ~24 µm to nanometers, and the average nano-hardness increased from 1.02 GPa to 2.82 GPa. Amorphous pure magnesium structure and body-centered cubic (BCC) lattice pure magnesium were reported. Molecular dynamics (MD) simulation confirmed that the high shear strain and hydrostatic pressure during ultra-precision cutting was the origin of amorphization and lattice transformation. The amorphous phase and a significant number of long-period stacking-ordered (LPSO) phases inside the pure magnesium were responsible for the high hardness after ultra-precision cutting.