In a novel plasma-shade reactor for oxidative reforming of biogas (CH 4 /CO 2 = 3/2), the effects of specific-energy-input (SEI) on CH 4 and CO 2 conversions and energy cost of syngas were investigated at O 2 /CH 4 ratios ranged from 0.42 to 0.67. At each of O 2 /CH 4 ratios, V-shape profiles of energy cost of syngas increasing with SEI were observed, reaching the lowest value at the optimal SEI (Opt-SEI). With the increase of O 2 /CH 4 ratio, the Opt-SEI decreased significantly. Moreover, at the Opt-SEI, O 2 and CH 4 conversions and dry-basis concentration of syngas increased and energy cost of syngas decreased greatly with the increase of O 2 /CH 4 ratio.