The invention discloses a preparation method of a 1, 3-alkyl tetrahydrothieno[3, 4-d] imidazole-2(3H)-4-diketone compound. The method comprises the following steps of: (1) under an alkaline condition, performing a reduction reaction on a 3-alkyl-5-oxo-6-alkyl-dihydroimidazo[1,5-c] thiazole-7-carboxylate compound and zinc powder, and performing post-treatment to obtain a 1, 3-dialkyl-2-oxo-5-sulfydryl-imidazoline-4-carboxylate compound; and (2) performing a dehydration and cyclization reaction for the 1, 3-dialkyl-2-oxo-5-sulfydryl-imidazoline-4-carboxylate compound in the presence of a catalyst and an organic solvent, and performing post-treatment to obtain the 1, 3-alkyl tetrahydrothieno[3,4-d] imidazole-2(3H)-4-diketone compound. The method is low in production cost and environment-friendly, and meanwhile, the reaction condition is mild and easy to control, the yield is high, and the product is easy to separate.