High-voltage direct current gas-insulated switchgear (DC GIS), as a critical power equipment in DC power grids, boasts high reliability, large transmission capacity, and minimal susceptibility to external environmental influences. It can significantly reduce the volume of both offshore and onshore converter stations, demonstrating vast application potential in DC power transmission systems. However, surface charges accumulation on gas-solid interfaces of insulators within DC equipment plays a pivotal role in causing electric field distortions and surface flashovers, which will leads to insulation failure. This study focuses on measuring the surface charge distribution of an real-sized insulator to derive its charge patterns. Subsequently, we delve into the impact of bipolar charge spots on surface discharge characteristics by examining discharge pathways and charge mappings. Our findings reveal that accumulated charges can be classified into four distinct patterns. Specifically, deposited positive charges hinder the progression of surface discharges, whereas negative charges extend the length of streamers. Polarity reversal leads to a significant increase in streamer length, which further reduces surface flashover.