Abstract: The magnetic field of electromagnetic separator originates from excitation system.For a long time, excitation system is adopted only as power supply device, ignoring the influence on magnetic field characteristics and ore dressing index because of its type.In this paper, three excitation systems are designed to supply power to electromagnets, the transient magnetic field variations are monitored using a high frequency Gauss meter, low grade ore dressing are tested by three excitation systems connect with Ф200.The test results show that the magnetic field waveform, the fluctuation range of magnetic field and the fluctuation frequency are obviously difference and increases with the increasing magnetic field fluctuation range and frequency.Then it is concluded that the power supply system of the magnetic separator is one of the important factors affecting the beneficiation index.