Thermal plasma pyrolysis can realize direct synthesis of acetylene and carbon black from methane, while the yields of products vary according to the reaction conditions. In the present work, a pre-proposed detailed kinetic mechanism is applied to investigate the efficiency and product distribution of methane pyrolysis in thermal plasma. The kinetic mechanism is validated with experiments, which proves the reliability of the mechanism under the conditions of thermal plasma. From simulation results, higher temperature and residence time in millisecond are beneficial for higher acetylene yield, while long residence time is beneficial for the production of carbon black. To be specific, when the temperature is higher than 2100 K, conversion of methane is above 99 wt % and the yield of acetylene is above 80 wt%, which is referred to as the selective synthesis of acetylene from methane. When temperature increases, the proper residence time for the maximum acetylene yield would decrease to within one millisecond.