The effects of water vapor on the adiabatic flame temperature, premixed flame temperature and laminar premixed flame velocity of methane combustion were studied by using Chemkin 17.0, and the effect of water vapor on the chain reaction process of methane combustion was also studied. The results show that with the increase of water vapor content, the temperature of methane flame and premixed laminar flame speed decrease, and the concentration of H,O,OH free radicals is also reduced, while the proportion of OH free radical is raised, leading to the dominance of reactions transmitted by OH free radicals. The addition of water vapor changes the chain reaction of methane combustion, enhances the reaction of CH3 to CH2(s),CH2(s) to CH2,CH2 to CH,CH to CH2O, and enhances the reaction of CH3 to CH3O,CH3O to CH2O at the same time.