Using a LUMEX RA-915+ mercury vapour analyser, field measurements were performed to determine the gaseous mercury (Hg) concentrations in the soils of the North China plain, Loess Plateau, Changping District, Haiyuan fault zone, and north Beiluntai (BLT) fault at the edge of south Tianshan. The factors affecting the soil gas Hg concentration, such as the sampler type, test hole depth, soil characteristics, and fault characteristics, and the mechanisms of their influence are discussed in terms of a gas diffusion equation. The results show that (1) the soil gas Hg concentration is mainly affected by the time lapse between the hole drilling and measurement, test hole depth, and sampler shape; (2) the measured soil gas Hg concentration agrees well with the analytical solution of the gas diffusion equation, and the curve shape is closely related to the degree of Hg enrichment in the soil and the soil density; and (3) the soil gas concentration in the fracture zone is largely affected by rock type, tectonism, the fault slip rate, the degree of fault locking, the development degree of fractures between the hanging wall and footwall, and the degree of fracture locking because these factors can alter the Hg upward transport channels and degree of surface enrichment. The effects of these factors on the diffusion coefficient in shallow soil layers are insignificant. The diffusion coefficient D depends on the particle size, density and porosity of the soil materials.