Abstract: The Fe-0.6C-24Mn-5Al high manganese austenitic steel was designed and fabricated and then subjected to a high temperature solution treatment at 1200 ℃ for 2 h. The impact toughness and impact deformation microstructure of the steel at different temperatures （room temperature, −115 ℃, −196 ℃, and −269 ℃） were investigated. The results show that the steel exhibited excellent impact toughness at all temperatures, with impact energy absorption ranging from 190 to 260 J. The fracture mode was mainly shear fracture, with dimples elongated by shear and crack propagation 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 deformation twinning. At −115 ℃, the plastic deformation mechanism was mainly activated by a single twinning system, with a small deformation twin spacing, significantly shortening the dislocation slip distance and strongly interacting with dislocations. At −196 ℃, the plastic deformation mechanism was dominated by intense planar dislocation slip, and the double cross-slip associated with the slip bands promoted dislocation proliferation and homogenization of plastic deformation, alleviating local stress concentration. At −269 ℃, a composite structure of nanoscale deformation twins and high-density dislocation slip bands coexisted in the deformed microstructure.