The effects of arc discharge mode on biogas reforming performance in an AC-pulsed arc plasma system were investigated for different pulse rising times. The discharge mode changed from the re-strike arc mode to constant arc discharge mode on increasing the pulse rising time from 2 to 5 mu s at a fixed discharge power. The length of the arc column periodically changed under re-strike mode, while the arc column was always longer under constant arc mode. The production yield of synthesis gas and the energy efficiency significantly improved when the arc discharge was operated under constant arc mode. The conversion rates of CH4 and CO2 increased from 15 to 80 and 12 to 27%, respectively. Under constant arc mode, the production yield of synthesis gas was 6.5 times higher than that under re-strike arc mode. In addition, the energy cost for synthesis gas production decreased remarkably from 52 to 9 kJ/mol. This is because the biogas reforming reactions, including thermal cracking, dry reforming, and partial oxidation, proceeded effectively in an area of the arc plasma, wherein the gas could react in the high-enthalpy region for a duration sufficient to ensure efficient reforming reactions. Therefore, the arc discharge mode significantly affected the chemical reactions and energy efficiency of biogas reforming.