Abstract: Fe₄₀Cr₄₀Ni₂₀ multi-principal element alloy was prepared using vacuum induction melting, cold-rolling, and annealing. The transformation of the alloy’s microstructure and its cryogenic mechanical properties at 77 K were investigated by X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and tensile testing. The results indicated that the solid-solution alloy possesses a dual-phase heterostructure with face-centered cubic (FCC) and body-centered cubic (BCC) phases. After the cold-rolled and annealed treatment, the original FCC phase transformed into a heterostructure composed of fine FCC grains and BCC grains, while the original BCC phase evolved into a heterostructure consisting of non-recrystallized BCC matrix and short lamellar FCC phases. The cryogenic mechanical properties of the alloy were significantly enhanced after cold rolling and annealing, resulting in a yield strength of 1 318 MPa, an ultimate tensile strength of 1 558 MPa, and an elongation of 24.5%. The strengthening of the alloy is primarily attributed to ​the obstruction of dislocation motion by various interfaces, including the heterogeneous FCC/BCC interfaces, grain boundaries, and twin boundaries. The deformation mechanism of the alloy is mainly dislocation slip, supplemented by twinning.