The invention discloses a flat steel ingot for a corrosion-resistant mold and a process method thereof, belonging to the technical field of metallurgical production process. To provide a flat steel ingot for corrosion-resistant molds with high strength, corrosion resistance, wear resistance, creep resistance, and fatigue resistance, and the application also provides a process for producing the flat steel ingot for corrosion-resistant molds . The flat steel ingot is a metallurgical die-cast steel ingot containing the following components in parts by weight: C 0.25-0.65%, Si 0.4-0.8%, Mn 0.4-0.9%, Cr 12-18%, Ni 0.05-0.3% , Mo 0.1-0.3%, N 0.06-0.30%, Al≤0.02%, P≤0.020%, S≤0.005%, and the rest are Fe and unavoidable impurity elements. The process method includes several steps of smelting molten steel with a nitrogen content less than or equal to 45ppm, refining nitrogen-rich and manganese-rich molten steel, and die casting, wherein the refining of nitrogen-rich and manganese-rich molten steel includes large slag in a refining furnace Quantitative refining and secondary refining in vacuum refining furnace.