The invention discloses nickel-free high-molybdenum anti-corrosion cast steel and a preparation method and application thereof. The nickel-free high-molybdenum anti-corrosion cast steel includes the following components, by mass, no more than 0.15% of C, 12.4%-12.9% of Cr, 4.5%-5% of Mo, 0.3%-0.5% of Si, no more than 0.1% of Mn, 0.5%-1% of Cu, no more than 0.1% of Ti and the balance Fe, wherein the mass ratio of Cr and Mo is 2.5-2.6:1; and the nickel-free high-molybdenum anti-corrosion cast steel is austenite complex phase steel. The preparation method includes the steps that after materials are taken according to the designed components, smelting and drossing are conducted and then casting is conducted; and an as-cast product is obtained. The as-cast product has an excellent anti-salt corrosion performance and is suitable for being applied to engineering devices represented by roots blowers in severe environments such as high temperature environments and salt mist environments. The components of the nickel-free high-molybdenum anti-corrosion cast steel are reasonably designed; compared with the same type steel alloy materials, the price is cheap, the nickel-free high-molybdenum anti-corrosion cast steel is suitable for large-scale industrialized application.