A tripod, a vertical coil and a total-field magnetometer are utilized to measure a geomagnetic field T0 without an additional magnetic field imposed, as well as two composite magnetic field values T−1 and T−2 when the geomagnetic field is added with a vertical upward magnetic field Tf and a double vertical upward magnetic field 2Tf, respectively. Calculate a vertical component Z, a horizontal component H and a geomagnetic inclination I of the geomagnetic field. Set up a horizontal coil such that a geometric center of the horizontal coil coincides with a geometric center of the vertical coil. Use the total-field magnetometer to measure two composite magnetic field values T+∥ and T−∥ after the geomagnetic field has been added with a horizontal forward magnetic field and a horizontal reverse magnetic field, respectively. Calculate a geomagnetic declination D.