This study introduces the principle and method for the use of differential global positioning system (GPS) in measuring azimuth in the China Geomagnetic Field Monitoring Network (CGFMN). We also present some azimuth measurement results and accuracy levels by geomagnetic observatories and geomagnetic survey stations, and offer some suggestions on how different geomagnetic observatories can measure azimuth in the future. We adopted two sets of differential GPS (model PROMAK 2 and PROMAK 100) including auxiliary devices antennas, cables, connected bases, and used one theodolite (model Mingeo) with an accuracy higher than 1 second. We installed the two differential GPS on two tripods, locating one near a mark (called station A) and the other alongside a fixed station room (station B) to measure the angle between geographical north (N) and a line AB (∠NAB).The entire measuring process ensures that the antennas and tripods remained stable and that the distance between stations A and B was more than 200 m. The GPS can receive satellite data until the transmission distance is more than 5 km, then the system turns off, and the above procedure is repeated. ∠NAB is computed using the two GPS data files using a special program. The theodolite was installed on a tripod near the mark to measure the angle between lines AP and AB (∠PAB, P is one of pillars), then it was moved to the pillar to measure the angle between the lines PA and PM (∠APM, M is one of marks). As such, the azimuth between geographical north (N) and the line from any pillar to any mark is determined through known angles. There are several methods for measuring azimuth. The proposed CGFMN azimuth measurement has the accuracy of the astronomical and differential GPS methods, less than 6″ of arc, which is higher than that of other equipment. Since 2001, the Lanzhou geomagnetic observatory has used differential GPS to measure the azimuth of 14 geomagnetic observatories and more than one thousand geomagnetic survey stations. Currently, the differential GPS method has replaced the CGFMN's astronomical method of azimuth measurement, using extensive GPS application. The main advantages of the differential GPS method over the astronomical method is that, in addition to having the same accuracy, it is free from the effects of weather conditions and human factors, all equipment can be operated easily by observers, and the data processing is completely computerized and has higher efficiency.