when the sample has an external magnetic field in the superconducting ring axis of the superconducting quantum interference device, its weak change will induce a ring current in the ring. The external magnetic field of the sample is measured by the change of the ring current caused by the change of the external field in the sample. The signal is directly measured by the superconducting ring of the magnetic flux converter and coupled to the superconducting ring. The signal in the superconducting ring is input into the processing circuit working at room temperature for amplification, filtering and comparison, and then converted into the required magnetic moment (three orthogonal components), magnetic inclination, magnetic declination, etc.