Abstract: To explore the effect of Ni content on the microstructure and mechanical properties of Fe-Mn-Al-C-Ni alloys after solution treatment and aging treatment, two types of austenitic low-density steels with Ni mass fractions of 5% and 10% (denoted as 5Ni and 10Ni) were selected. The precipitation behavior of B2 phase and mechanical properties of the alloys were systematically investigated via scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), room-temperature tensile tests, and phase diagram calculations. The results show that the solution-treated alloys are composed of an austenite matrix and micron-scale B2 phase distributed along grain boundaries. With the increase of Ni content, the volume fraction of B2 phase along grain-boundary increases from 5% to 22%, which strengthens the pinning effect on austenite grain boundaries and refines the matrix grain size from 11.5 μm to 4.3 μm. After aging treatment, nanoscale needle-like B2 phases precipitate within the austenite matrix of both two alloys. The increase in Ni content significantly improves the nucleation rate of B2 phase, achieving a more uniform distribution and finer grain size of the precipitated phase. The plastic deformation mechanism of both alloys is dominated by planar slip. Due to the non-shearable characteristic of B2 precipitates, the Orowan bypass mechanism is activated to interact with dislocations, which endows the aged alloys with excellent work hardening performance. Although the increase of Ni content refines grains and promotes the precipitation of needle-like B2 phases, a larger number of coarse grain-boundary B2 phases in the 10Ni alloy significantly deteriorate its plasticity. Accordingly, the 5Ni alloy presents the optimal strength-ductility matching, with a tensile strength of approximately 1 220 MPa and a uniform elongation of around 36%. This study provides a theoretical basis for regulating the microstructure and properties of austenitic low-density steels through alloy composition design.