As the main component of natural gas and an important chemical raw material, methane easily forms a mixture of flammable gases during production and processing. Therefore, understanding the flammability limit and limiting oxygen concentration of methane is very important in predicting the possibility of fire and explosion in industrial production and designing the corresponding monitoring parameters. To solve this problem, the effects of the addition of a C2H6/C2H4/CO/H-2 mixture on the flammability limit of methane and the limiting oxygen concentration under N-2 dilution were systematically studied. In addition, the flammability limit was evaluated using the limiting burning velocity theory and detailed reaction kinetic model, and a sensitivity analysis was conducted to study the chemical kinetics near the limit. The experimental results show that when the volume fraction of mixed gas gradually increases, the upper and lower flammability limit of methane both decrease, and the decrease in the lower flammability limit is larger than that of the upper flammability limit. Under the condition of nitrogen dilution, the limiting oxygen concentration gradually decreases with an increase in mixed gas concentration. The explosion triangle expands and moves to the lower-left corner, the flammability limit range increases, and a greater danger of explosion exists. The calculated results based on the limit laminar burning velocity are in good agreement with the experimental data. The sensitivity analysis shows that elementary reactions involving the active free radicals of OH, H and O have higher sensitivity. With the addition of mixed gas, the chain-branching reaction R31 is promoted, the chain termination reactions R12 and R41 are inhibited, the competition between the chain-branching reaction and chain termination reaction changes, and the flammability limit range of methane expands. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.