Fluxgate magnetometer is a device for the measurement of weak magnetic field

and it can be used to measure geomagnetic fields.The structure and operation principle of the magnetometer are introduced.Three-dimensional spherical feedback coils system accomplishes the measurement of three components of geomagnetic field based on the one dimensional open loop system which has been successfully used to get the relative measurement of the earth magnetic field.It shows that half day is the major period of the geomagnetic field in the measurement direction based on one dimensional system.The day fluctuation of geomagnetic field is less than（3 μT） based on the data from three dimensional system.