The invention discloses an air-ground integrated geomagnetic field combined observation method and system. The method comprises the following steps of validating local geomagnetic field model parameters using the geomagnetic field data collected by a mobile earth station before the geomagnetic field survey mission is executed; getting a dynamic model along the flight path using the geomagnetic field data collected by the airborne console over the flight path and the model parameters when the geomagnetic field survey mission is executed; getting a measuring error model along the flight path using the attitude measuring error obtained by the airborne console when the aircraft flies as per the preset attitude; getting the estimation of the geomagnetic field data collected over the flight path in a geographic coordinate frame using the dynamic model and the measuring error model. In the invention, the measuring accuracy of a geomagnetic field vector expressed in the geographic coordinate frame can be improved, and the geomagnetic field anomaly detection can be implemented.