The invention provides a method for controlling the particle size of nano-powder. According to the method, only a superconducting magnetic field with the magnetic density being 0.1 to 14 T is required to be imposed to the evaporation-condensation part without changing the preparation device during the process of preparing the nano-powder based on the evaporation-condensation method which takes resistors, high-frequency induction, plasma, electron beams, laser and the like as the heating source. Since the magnetic field changes the critical nucleation energy of the evaporated atoms and further changes the nucleation concentration and the nucleation speed, thereby achieving the purpose of controlling the particle size of the nano-powder through adjusting the magnetic intensity. The method improves the efficiency, the uniformity and the particle size adjustment range of the nano-powder preparation based on the evaporation condensation method.