According to computation results of C-K equilibrium systems, C2H2 and C2H are the main hydrocarbon in the C-H equilibrium system at the temperature of approximately 3500 K. Because hydrogen plasma has the advantage of high temperature (over 3500 K), acetylene can be directly produced by coal pyrolysis in hydrogen plasma. In order to obtain high yields of acetylene, a quenching process is needed to fix the acetylene produced at high temperature. It is proved that an adequate quenching rate (0.77 similar to 5.8 x 10(8) K/s) can avoid the decomposition of acetylene, but will not prevent C2H radicals recombining into acetylene [Chem. Eng. Sci. 54 (1999) 957]. A dynamic chemical method is employed in this paper to study the mechanism of C2H radicals' recombination into acetylene in the quenching process. Primary experiments have also been carried out to study the process of coal pyrolysis in hydrogen plasma. It is shown by the calculation results that: (1) the reaction that really has an effect on acetylene yield in the quenching process is the recombination of C2H and H-2, and not that of C2H and H in traditional opinions; (2) if-the recombination of C2H and H-2 is taken into account, the total mass content of acetylene in the quenched gas may increase from 58% to 78% at the quenching rate which can prevent acetylene from decomposing. The experimental results prove that C2H radicals really recombine into acetylene in the quenching process.