The development caurse of research on the precursor characteristics of natureal geoelectric field before earthquakes in Chinese mainland is reviewed firstly. Then the range, figure and period of the field, as well as the statistical relationship with earthquake magnitudes are discussed. Some possible mechanisms of natural electric field before earthquake, such as infiltration electrical field, diffuse-adsorption potential, electro-chemical potential, electro-mechanical transformation effect and natural self-potential variety of loess under pressure, are pointed out. The natural electric field is a summating potential （SP） made of many signals, the trend background, seasonal variation, seismic signal and interference factors can all cause the change of rock resistivity, ionization and solubility of solution in soil, permeability condition of underground water, also the geoelectric field and natural electric field charge. The research on these mechanism of anomalies and characteristics of electromagnetic response will help us to understand the earthquake precursors.