In this paper, based on fluxgate technology and zero flux principle, an AC / DC sensor is studied. The magnetic field generated by the primary and secondary currents is gathered and superimposed by the poly-magnetic ring. The magnetic field is detected by a high-sensitivity fluxgate sensor embedded in the polygonal ring, and then outputted through a high-gain and high-bandwidth power amplifier circuit. Thereby a secondary current corresponding to the primary current is produced. Primary current and secondary current generated the magnetic field the values are equal and opposite in direction in the magnetic ring, and ultimately make the magnetic field in the magnetic ring is zero, Primary current and secondary current ampere turns ratio is equal. At the same time, two fluxgate sensors symmetrically distributed on both sides of the magnetic ring are used to improve the anti-interference ability of the system to the external magnetic field. Experiments show that the current sensor of the AC-DC proportional error is better than 0.01.