Surface flashover occurs at the external insulation of epoxy-based dry-type air-core reactors, which suffers from severe electric field distortion at the interface between the star-frame arm and epoxy/glass composites. A study on the triggering factors of ac and dc surface flashovers and their effects on the characteristics of ac/dc superimposed flashovers is urgently needed. In this study, the characteristics of surface flashover, partial discharge (PD), and surface charge were measured on the surface of epoxy composites using dc voltage, ac voltage, and ac/dc superposition with different voltage ratios. The PD intensity increases, the surface charge density decreases, and the flashover voltage lowers as the ac content of the ac/dc superimposed voltage increases. When the component of dc increases, the trends of flashover voltage, PD, and charge density are opposite to those for the ac component. According to the link between the three components, surface charge is the triggering issue of dc flashover, but PD rather than surface charge accumulation is the dominant factor of ac flashover. The main effect of surface charge accumulation is to distort the electric field at the high-voltage (HV) electrode's triple junctions (HTJs) and generate more initial electrons under dc voltage, while the main effect of PD in ac flashover is to directly intensify the gas discharge and generate more electrons. This study reveals the predominant triggering factors of ac and dc flashover and their impact mechanisms on ac/dc superimposed surface flashover, providing theoretical guidance for subsequent enhancement of ac/dc superimposed flashover characteristics of power equipment.