Abstract: Weak magnetic field detection has played an increasingly important role in the field of scientific research, industrial manufacture, and people's daily lives. In recent years, a new kind of magnetic sensor has been proposed, composed of a superconducting flux-to-field transformer and a high-sensitivity magnetoresistive sensor. This kind of sensor is expected to be widely used because of its excellent sensitivity (femtotesla), stability, band characteristics, and low cost. In this paper, its structure and principle of operation will first be reviewed. We then describe and compare the development and applications of giant magnetoresistance sensors and the tunnel magnetoresistance/superconductor based mixed sensors fabricated in our lab.