The invention discloses a preparation method of a composite rare earth oxide reinforced molybdenum alloy piercing mandrel. The method includes the steps: firstly, uniformly mixing carbon powder, titanium hydride powder, zirconium hydride powder and rare earth oxide powder in a three-dimensional mixer to obtain doped powder, and then uniformly mixing the doped powder and molybdenum powder to obtain mixed powder raw materials; secondly, pressing the mixed powder raw materials to form piercing mandrel blanks; thirdly, finishing the piercing mandrel blanks to remove redundant piercing mandrel blanks; fourthly, sintering the finished piercing mandrel blanks, and cooling the sintered the piercing mandrel blanks to reach indoor temperature to obtain the composite rare earth oxide reinforced molybdenum alloy piercing mandrel. Composite rare earth oxide is added based on a TZM (titanium zirconium molybdenum) alloy, and the molybdenum alloy piercing mandrel with fine high-temperature hardness and flexural toughness is prepared by the aid of technological processes such as mixing, pressing, finishing and sintering and can stably adapt to high-temperature, high-pressure, splat cooling and quick heating working environments.