Abstract: Issues of insulator surface flashover often cause the failures of electromagnetic pulse simulators. By coupling the continuity equations of positive ion, negative ion and electron in the discharge with poisson equation, a simulation model of flashover along gas-solid interface under voltage pulses has been established. The initiation and development characteristics of flashover streamers of needle-plane geometry under positive and negative voltage pulses have been simulated in various gas pressures respectively. By comparing the simulation results with the experiment results, the effects of voltage polarity and gas pressure on flashover streamers have been analyzed. The results show that the drastic ionization happened in the area of positive streamer head whereas the negative-directed positive streamer happened under negative pulses. The surface charge accumulation of negative streamers is more drastic than the positive one, which is the main reason for the formation of cathode-directed positive streamers. The simulation results also show that as the gas pressure rises, the steamer radius descends and the accumulation of surface charge increases.