The invention relates to a preparation method of a steel-making additive for smelting molybdenum-bearing steel and belongs to the technical field of smelting of molybdenum-bearing steel. In view of the problems of high cost of ferro-molybdenum and volatilization of molybdenum trioxide at high temperature, the invention provides a novel additive-molybdenum dioxide, used for smelting molybdenum-bearing steel. The preparation method is characterized in that industry molybdenum trioxide obtained through calcinations of molybdenum concentrate is taken as the raw material, and industry molybdenum dioxide is prepared via carbon addition reduction. Compared with molybdenum trioxide, molybdenum dioxide has the advantages of no volatilization at high temperature, high molybdenum content, high density and the like. Compared the prior art, the preparation method overcomes the shortcomings that ferro-molybdenum is high in cost, and molybdenum trioxide is volatile when molybdenum trioxide is directly used for alloying steel-making, and has the characteristics of high molybdenum yield, remarkable economic benefit and the like.