The utility model discloses a testing device of a silicon steel iron core ultimate hysteresis loop. The testing device of the silicon steel iron core ultimate hysteresis loop comprises an autotransformer, a customized iron core, a primary coil, a secondary coil, a wave recording instrument, a current probe, a voltage probe and a current-limiting resistor, wherein the primary coil and the secondary coil are wound around two sides of the customized iron core, a high-voltage end of the autotransformer is in external connection with an alternating current power supply, the primary coil is connected with a low-voltage end of the autotransformer after being connected with the current-limiting resistor in series, a clamp type input end of the current probe is clamped on a current loop of the primary coil, an input end of the voltage probe is connected with the secondary coil, and an output end of the current probe and an output end of the voltage probe are respectively connected with a wave recording channel of the wave recording instrument. The testing device of the silicon steel iron core ultimate hysteresis loop has the advantages of being simple in structure and low in cost, is suitable for various experiment institutions and enterprises with needs for obtaining feature curves of ultimate hysteresis loops of silicon steel iron cores, and has a good application prospect.