The invention relates to a marinated tofu coagulant and a preparation method thereof. The marinated tofu coagulant is prepared from the following raw materials in parts by weight: 80-90 parts of watersqueezed out from tofu, 5-10 parts of an active lactic acid bacteria solution, 10-20 parts of soluble starch and 10-15 parts of gluconolactone. The preparation method includes the steps: 1) production of the active lactic acid bacteria solution; 2) mixed fermentation; and 3) finished product obtaining. Squeezed water produced in the process of tofu pressing is adopted, self-made vegetable pickling water solution rich in active lactic acid bacteria is added as a lactic acid bacteria fermentation strain, fermentation is performed, the traditional magnesium chloride and calcium sulfate halide are eliminated, and the bitter and astringent taste is eliminated; at the same time, trace amino acid produced by the lactic acid bacteria in the tofu water and vegetable pickling water fermentation process makes tofu have strong flavor. With increase of soluble starch and gluconolactone, the taste of the tofu can be improved, and the mouthfeel of the tofu becomes tender and smooth. The tofu prepared by the marinated tofu coagulant has no bitter and astringent taste of a traditional marinated tofu but also ensures tender and smooth taste.