Stainless steels are used in a wide range of complex environments due to their excellent corrosion resistance. Multiphase stainless steels can offer an excellent combination of strength, toughness and corrosion resistance due to the coexistence of different microstructures. The microstructure and mechanical properties of a novel cast multiphase stainless steel, composed of martensite, ferrite, and austenite, were investigated following appropriate heat treatment processes: solution treatment at 1,050 °C for 0.5 h followed by water quenching to room temperature, and aging treatment at 500 °C for 4 h followed by water quenching to room temperature. Results show reversed austenite is formed by diffusion of Ni element during aging process, and the enrichment of Ni atoms directly determines the mechanical stability of austenite. The austenite with a lower Ni content undergoes a martensitic transformation during plastic deformation. The tensile strength of the specimen exceeds 1,100 MPa and the elongation exceeds 24% after solid solution, and further increases to 1,247 MPa and 25% after aging treatment. This enhancement is due to the TRIP effect of austenite and the precipitation of the nanoscale G-phase pinning dislocations in ferrite and martensite.