The invention relates to a preparation method of methimazole. The method specifically comprises the following step: enabling reaction of N-methylimidazole, N-butyllithium and powdered sulfur in an organic solvent to obtain methimazole. The preparation method disclosed by the invention is simple in process, mild in condition, simple to operate, easy for post treatment, applicable for industrialized production, and capable of greatly improving the reaction yield of methimazole.