Molecular dynamics simulation （MD） was used to investigate the microscopic mechanism of martensitic transformations in Fe single crystals driven （along ［001］ orientation） by an accelerating piston. Simulated results revealed that the above compression process can be divided into five stages, namely the elastic compression, softening of elastic ratio, phase transformation （bcc to hcp）, over-relaxation of stress and elastic compression of high-pressure phase. The slipping laws of atoms and properties of stress are analysed in detail. Atoms can only slip along longitudinal direction under elastic compression, and when the longitudinal stress is beyond 10 GPa, local atoms begin to slip along lateral direction and form embryos, which is just the microscopic nature of the softening of elastic ratio.The stacking fault （fcc） is a more steady origin of crystal nucleation than the twin boundary. After the phase transformation, the atoms undergo an over-relaxation of stress （i.e., the lateral stress is larger than the longitudinal stress）, and this state disappears when the longitudinal stress is beyond about 36 GPa.