The present disclosure relates to a high-strength Fe—Cr—Al—Ni multiplex stainless steel and a manufacturing method therefor. The multiplex stainless steel comprises 35 to 67 wt % of iron (Fe), 13 to 30 wt % of chrome (Cr), 15 to 30 wt % of nickel (Ni), and 5 to 15 wt % of aluminum (Al) and has a multiplex structure in which an austenite phase accounting for high ductility, a ferrite phase accounting for high strength, and an NiAl(B2) phase providing both strength and high-temperature steam oxidation resistance, exist in combination. The multiplex stainless steel can secure necessary fabricability and mechanical strength even if for/in a thin state, can maintain integrity as a structural member in a normal operation condition of a light-water reactor thanks to the formation of a chrome oxide layer thereon, and can form a stable oxide layer including alumina under a high-temperature steam environment, which is plausible in a high-temperature nuclear accident, thereby providing exceptionally improved resistance to serious accidents.