In this paper, the influence mechanism of O2/H2O/CO2 on the removal of CH4 and NOx by non-thermal plasma (NTP) combined with selective catalytic reduction of NOx by CH4 (NTP-CH4-SCR) were investigated. The results show that the increase of NTP energy density (0 J/L-491 J/L) promoted the CH4 removal but inhibited the NOx removal in NTP-CH4-SCR. In CH4-SCR, H2O could significantly reduce catalyst activity, adding 0.5% H2O decreased the removal efficiency of CH4 and NOx from 30–2.9% and 58.4–1.8% respectively. In NTP-CH4-SCR, the increase of H2O content (0.5%-10.4%) contributed to formation of oxidizing free radicals such as OH and HO2, thus the CH4 removal efficiency increased from 24.1–37.4%. The increase of O2 content (0%-10%) promoted the adsorption of NO and the reaction of adsorption products with CH4 oxidation products, causing the CH4 removal efficiency increased from 1.3–32.3% and NOx removal efficiency increased from 1.5–61.6% in CH4-SCR. Increasing O2 (0–10%) produced more O and HO2 radicals in NTP-CH4-SCR, resulting in the increase of CH4 removal efficiency from 18.6–44.9%. However, these O and HO2 radicals would react with N radicals to form NO2, NO3 et al., thus decreasing NOx removal efficiency from 68–40.8%. The change of CO2 concentration has little effect on CH4 and NOx removal efficiency.