Through the field test method, G856T proton precession magnetometer is used to measure ferromagnetic substances of different mass at selected points in the field, and ferromagnetic substances of the same mass are placed at points in different directions in the field. GM4 fluxgate magnetometer is used to measure their interference with geomagnetic fields D, H and Z, combined with the typical examples of ferromagnetic substances interference in Tianjin regional geomagnetic station in recent years, the influence of ferromagnetic materials on geomagnetic observation is studied. The results show that: ① The measured results of the interference of ferromagnetic materials on geomagnetic observation are close to the theoretical calculated values in amplitude, and the deviation may be related to the magnetic inhomogeneity of underground media in the test site and the material, shape and volume of ferromagnetic materials. ② The greater the mass of ferromagnetic material, the wider the influence range of geomagnetic observation. The magnetic field gradient near ferromagnetic material is large, and the isoline becomes dense, and vice versa. ③ The interference amplitude and form of ferromagnetic substances on geomagnetic fields D, H and Z have certain rules in direction, which can be used as the basis for judging the orientation of ferromagnetic interference sources.