Negative abnormal changes of atmospheric electric field have been monitored many times before earthquakes, especially before large earthquakes. However, its mechanism has not been clearly concluded yet. Based on the comparative analysis of the positive anomaly characteristics of atmospheric electric field before lightning and geoelectric field before earthquakes in earthquake areas, this paper concludes that the negative anomalies of atmospheric electric field before earthquakes are caused by the rise of the geoelectric potential in earthquake areas, and the areas with the abnormal rise of the geoelectric potential are potential earthquake areas. Based on the inference above , this paper further proposes a comprehensive earthquake early warning method with station network layout by geoelectric field instruments in combination with the atmospheric electric field data monitored by Electric Field Mills (EFMs). The research results will provide a possible reference for exploring the mechanism of earthquake generation, and will be help to explore earthquake early warning.