CH4 oxidation in O-2/CO2/H2O environments was studied using reactive molecular dynamics (ReaxFF-MD) method with the aim to investigate the chemical effects of CO2 and H2O under different conditions. The influence of molar heat capacity and thermal radiation of CO2 and H2O were eliminated, which estimated that only chemical effects were taken into account. The results showed that high concentration of CO2 inhibited the reaction rate of CH4 at low temperatures, but advanced CH4 oxidation rate at high temperatures because of CO2 reactivity expressed by CO2 + H -> CO + OH (R1). The presence of H2O promoted the oxidation rate of CH4 even at low temperatures. Reacting with O and H radicals through H2O + O -> OH + OH (R2) and H2O + H -> H-2 + OH (R3) dominate the effect of H2O. Furthermore, the interaction between CO2 reactivity and H2O reactivity was studied in O-2/CO2/H2O combustion under fuel rich condition. It was shown that the presence of 30%H2O promoted the conversion rate of CO2 at relatively low temperatures by providing more H radicals for R1, therefore enhanced CO production. However, an inhibition effect of H2O on CO2 conversion rate was found at high temperatures. R3 tended to occur at high temperatures, which would compete with R1 and thus the production of CO was suppressed. The main elementary reactions for CO2 and H2O were studied by analyzing the computed trajectory of the systems during ReaxFF-MD simulations. Finally, the effects of oxygen concentration on CH4/O-2/CO2/H2O systems were also studied. The results showed that the effect of CO2 and H2O were less pronounced with increasing O-2 concentration.