The invention discloses a preparation method of a 7-alkoxy-3-phenyltetrahydroimidazo[1,5-c]thiazole-5(1H)-one compound. The method comprises the following steps: sequentially reacting a dihydroimidazo[1,5-c]-thiazole-5,7(3H,6H)-dione compound with zinc powder and alcohol, and post-processing after the reaction to obtain the 7-alkoxy-3-phenyltetrahydroimidazo[1,5-c]thiazole-5(1H)-one compound. The preparation method adopting the zinc powder to reduce has the advantages of mild reaction, few side reactions, good reaction selectivity, product quality control benefiting, high yield reaching above 90%, and realization of environmental pollution reduction and cost control benefiting due to easy control and recycling after the filtration and separation of the zinc powder.