Cast stainless steels with high strength and toughness are very important for industrial production, but the strengthening methods are very limited. In this study, a novel cast multi-phase stainless steel (CMPSS) containing ferrite, martensite and austenite was prepared and its strengthening mechanism was investigated. After solid solution and tempering at 520 °C for 4 h, the yield strength of the sample reaches over 800 MPa, and the total elongation exceeds 13%. This tempering strengthening is mainly achieved by precipitation of the nanoscale Laves phase pinning dislocations in ferrite. However, when prolonging the tempering time or increasing the tempering temperature, the Laves-phase particles precipitate and coarsen along the phase boundary, which seriously damages the strength and toughness of the sample. In addition, high-temperature tempering also leads to a decrease in retained austenite (RA) content by reducing the amount of C and Mn elements in the lath martensite and promotes the precipitation of the R phase (molybdenum-rich intermetallic compound) in ferrite, which is also detrimental to the mechanical properties of CMPSS.