The invention discloses a Fe-Mn-Cr-Ni-series medium-entropy stainless steel and a preparation method thereof. The chemical components of the medium-entropy stainless steel are Fe40Mn20Cr20Ni20, and the medium-entropy stainless steel is prepared from raw materials with different qualities of high-purity Fe, high-purity Ni, high-purity Cr and Fe-Mn binary steel with an atomic ratio of 1 to 1; a vacuum arc melting and a copper mold suction casting method are adopted for casting, and the technological process comprises the following steps of smelting, heat treatment, cold working and the like. According to the stainless steel and the method, the medium-entropy stainless steel is designed through parameter calculation, the structure of the medium-entropy stainless steel is a single-phase face-centered cubic crystal structure, the medium-entropy stainless steel not only has good room-temperature strength and plastic deformation capability, and has especially excellent mechanical property atlow temperature; in addition, compared with 304 stainless steel, the medium-stainless steel has more excellent corrosion resistance in a 3.5% sodium chloride solution, in the future, elements such ascarbon, silicon and aluminum can be added into the medium-stainless steel, the comprehensive performance of the material is further enhanced. Therefore, the medium-entropy stainless steel has a very wide application prospect in marine equipment and alpine regions as structural materials.