Abstract: A Fe-0.6C-24Mn-5Al high manganese austenitic steel with high stacking fault energy was designed and fabricated, and then was subjected to solid solution treatment at 1200 ℃ for 2 h. The impact toughness and impact deformed microstructure of the steel at room temperature and extremely low temperatures （−115, −196, −269 ℃） were investigated. The results show that the steel exhibited excellent impact toughness at at room temperature and extremely low temperatures, with impact energy absorption ranging from 190 J to 260 J. The fracture mode was mainly shear fracture, with dimples elongated by shear and crack propagation in grains in a serrated pattern. At room temperature, the plastic deformation mechanism of the test steel was dominated by planar slip, with a small amount of deformed twins. At −115 ℃, the plastic deformation mechanism was mainly activated by a single twinning system. The deformed twins had small spanning and could shorten the dislocation slip distance. At −196, −269 ℃, the plastic deformation mechanism was dominated by intense planar dislocation slip. At −269 ℃, a composite structure of nanoscale deformation twins and high-density dislocation slip bands coexisted in the deformed microstructure.