The invention relates to a preparation method of 2,3-dimercapto-6-methylquinoxaline, which comprises the steps of: dehydrating and cyclizing by using 4-methylphenylene-diamine and oxalate as raw materials in a solvent under the presence of an acid binding agent and then halogenating simulataneously, concentrating for recovering a halogenating agent and the solvent after reacting completely, neutralizing, filtering and drying to obtain 2,3-dichloro-6-methylquinoxaline; and carrying out mercaptolation reaction on the 2,3-dichloro-6-methylquinoxaline prepared by the steps with hydrosulfide salt in a strong polar solvent, then concentrating the solvent, neutralizing, filtering deposition, and drying to obtain the 2,3-dimercapto-6-methylquinoxaline. The 2,3-dimercapto-6-methylquinoxaline can be obtained by adopting the 4-methylphenylene-diamine as the raw material and using a one-pot method through cyclization and chlorination, which is convenient for production operation, thereby enablingthe preparation cost to be greatly lowered. The reaction integrates the cyclization and the chlorination, and has high reaction speed, short preparation period and simple reaction postprocessing, thereby greatly improving the working efficiency and the energy utilization.