A novel sensor for micro-magnetic field detection is proposed based on the physical characteristics and giant magneto-impedance (GMI) effect of Fe-Co-Si-B amorphous wires. The sensor circuit ensures high sensitivity, high detection precision, fast response and small power consumption. The signal processing circuit taking complex programmable logic device (CPLD) as core chip not only improves the identification ability for pseudo ferromagnetic objects, but also ensures the real-rime response. It is the foundation for the proposed sensor to serve widely in military and civil. By using its high sensitivity for geomagnetism variation, as an application example, the sensor is used to compare and analyze different cars.