The present invention belongs to the field of metallurgy of iron and steel, and is especially the method of treating molten steel with RE oxide during secondary steelmaking process. RE oxide or RE ore concentrate powder as RE source is mixed with reductant, additive and small amount of adhesive, and the mixture is produced into block at high pressure, crushed, formed and pelletized to form various kinds of RE oxide-based molten steel treating agent. In the conditions of depressing, blowing argon and reducing in RH, VD and other refining process, RE oxide is heat reduced to fed RE into steel. The C and Si containing forms of the RE oxide-based molten steel treating agent is suitable for molten iron altering treatment during nodular cast iron and compacted graphite cast iron production.The present invention is superior to direct RE adding method in smelting steel with high cost and more foreign matters.