The invention relates to a preparation method of a seawater-corrosion-resisting high-saturation-induction-intensity magnetically soft alloy. The alloy comprises the following components in percentage by weight: 18.0-25.0 percent of Cr, 0.5-3.0 percent of Mo, 0.16-2.8 percent of micro-alloyed elements and the balance of Fe. All the components are subjected to vacuum smelting, electro-slag remelting, forging and thermal treatment to obtain the magnetically soft alloy. The alloy satisfies the requirement on corrosion resistance under a severe seawater environment and is also excellent in magnetic properties, mechanical strength and seawater corrosion resistance and high in saturation induction intensity.