Abstract: The microstructure and mechanical properties of Fe-28Mn-6.2Al-1C-4Ni steel annealed at 650 ℃ and 750 ℃ for 0.5 h were studied by optical microscopy (OM), electron backscatter diffraction (EBSD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The stacking fault energy was calculated and the deformation mechanism was analyzed. The results show that the annealing temperature has a significant influence on the microstructure and mechanical properties of Fe-28Mn-6.2Al-1C-4Ni steel. The experimental steel after different annealing temperatures consists of austenite and annealing twins, and the grain sizes are 22.55 μm and 10.46 μm for the steel annealed 650 ℃ and 750 ℃, respectively. With the increase of annealing temperature, the strength and plasticity of the low-density steel increase together. After annealing at 650 ℃, the tensile strength is 819.2 MPa, the elongation is 52.1%, and the product of strength and elongation is 42.68 GPa·%; After annealing at 750 ℃, the tensile strength is 992.3 MPa, the elongation is 54.6%, and the product of strength and elongation is 54.18 GPa·%. After tensile deformation, micro strip, dislocation walls and Taylor lattices can be observed in TEM images of experimental steel, which can improve the strength and plasticity of low density steel. According to Olson-Cohen thermodynamics model, the stacking fault energies are 70.91 mJ/m² and 76.46 mJ/m² for low density steel annealed at 650 ℃ and 750 ℃, respectively. Combined with stacking fault energy and TEM images, the plastic deformation mechanism of the experimental steel is identified as dislocation planar slip.