The invention relates to a ferromagnetic element's iron core loss measuring method based on a random waveform low frequency power supply. The ferromagnetic element's iron core loss measuring method comprises the steps: 1) establishing a T type equivalent circuit taking regard of hysteresis and eddy current loss, for a ferromagnetic element; 2) applying a random waveform low frequency power supply to calculate the eddy current loss equivalent resistance Re; 3) drawing a limitation magnetization loop line according to the voltage and current under the iron core saturation condition; 4) multiplying the time integral value of the flux linkage on the magnetization loop line (the area of the magnetization loop line) by the frequency to calculate the hysteresis loss; and 5) calculating the eddy current loss through the power frequency induction electromotive force and the eddy current equivalent resistance, and adding the hysteresis and eddy current loss, and then obtaining the ferromagnetic element's iron core loss. The ferromagnetic element's iron core loss measuring method can complete the test when the frequency is far lower than the power frequency and the power frequency voltage, and has lower requirement for the test power supply waveform, and can greatly reduce the test equipment capacity, and can reduce the mass, and the volume of the test equipment, and can effectively reduce the safety risk for the staff and the tested products during the testing process, and has accuracy being identical to the power frequency test.