The invention provides a preparation method of 2-arylbenzimidazole, which comprises the steps of firstly adding ortho-phenylene diamine and benzoic acid or substituted benzoic acid into a reactor, then adding 1,4-dioxane, dropwise adding concentrated sulfuric acid while stirring, stirring for back flow for 3-5h, obtaining a mixed solution, cooling the mixed solution to the room temperature, adding a NaOH water solution into the mixed solution, allowing a solid to be precipitated in the mixed solution, conducting suction filtrationunder reduced pressure, drying, and obtaining 2-arylbenzimidazole. According to the method, 2-arylbenzimidazole is prepared by taking 1,4-dioxane as a solvent and concentrated sulfuric acid as a catalyst and adopting a one-pot method; concentrated sulfuric acid is cheap, easy to get and good in catalytic effect; therefore, the method is simple in operational process, low in cost, and short in reaction time. In addition, with the adoption of the method, as high yield of 2-arylbenzimidazole can be realized without recrystallization, the post-treatment is simple.