The invention provides a food-grade dishwashing liquid and a preparation method thereof. The dishwashing liquid can thoroughly remove pesticide residues, dust, oil stains, insect stains and other harmful substances on surfaces of vegetables and fruits. Raw materials are all food-grade raw materials, chemical surface activity auxiliary chemicals, synthetic essence and the like are not added, secondary pollution is avoided, and the dishwashing liquid is water-saving and environment-friendly during usage. Vessels cannot be scratched when the product is used for wiping things, the dishwashing liquid has the characteristics of quickly cleaning kitchenware or tableware stained with oil stains, saving water and the like, and repeated washing is not required; a sargassum fermented product, a pomelo peel extract and a brewing mass water extraction liquid are used as main bactericidal ingredients, and all components have synergistic interaction to realize the bactericidal effect. The dishwashing liquid is low in cost, simple and easy to prepare, convenient to use, non-toxic and non-irritating to human bodies, high in biodegradability, free of secondary pollution and environment-friendly.