Abstract: Mercury is mainly from deep mantle rocks and magma in the Earth and has low solubility, low vapor pressure, and strong penetrating ability. Before earthquakes, mercury can pass through different media to reach the surface under high temperatures and large pressure gradients, causing pre-earthquake mercury anomalies. The precursory anomalies of mercury are usually used as an important indicator for earthquake prediction. In this paper, we comprehensively analyze the domestic literature which uses mercury for seismic observations. We obtain the following conclusions: (1) Compared to gaseous mercury, water-soluble mercury can be collected more conveniently, is less susceptible to interference from transmission media, and has larger precursor anomalies. Hence, it is a better indicator of pre-earthquake observations than the gaseous mercury. (2) There is a big interval between the end of water-soluble mercury anomaly and the start of earthquakes, while gaseous-mercury anomalies usually last until the earthquakes start. (3) The anomalous concentration of mercury before earthquakes is usually several to dozens of times of the background value. The concentration usually depends on the distance to the epicenter, earthquake magnitude, and geological structure. (4) For earthquakes with magnitude less than 5, the water-soluble mercury anomalies are usually found within a radius of 200 km around the epicenter, while for earthquakes with magnitude between 5 and 6, the radius is less than 500 km. In addition, the mercury anomalies before earthquakes are usually related to the underground fluid activity under the action of the seismogenic stress field. The isotope fractionation technology of mercury provides a new method to study the source of mercury in the fault zone and to explore the relation between mercury and fluid activity in the deep crust.