The invention relates to martensite steam turbine blade steel, in particular to molybdenum-cerium composite alloyed steam turbine blade steel and a preparation method thereof, and belongs to the field of steel and iron materials. Molybdenum and cerium are synchronously added in 1Cr13 steel; the addition of cerium is to improve the activity of molybdenum in the steel and to enhance the effect of molybdenum; cerium also can improve high-temperature mechanical performance and corrosion resistance of the steel; and cerium can improve high-temperature oxidation resistance of the steel to achieve the composite alloying effect. Compared with steel only added with a Mo element, the molybdenum-cerium composite alloyed steam turbine blade steel is low in added cerium element amount and low in cost increment, but enhances the effect of molybdenum, and improves the high-temperature oxidation resistance. Compared with steel singly added with a rare earth element, the molybdenum-cerium composite alloyed steam turbine blade steel is more excellent in high-temperature mechanical performance and corrosion resistance.