The invention relates to a preparation method of a sulforaphane capsule. The method comprises the following steps: (1) dehydrating, drying and smashing a flowering branch of an edible part of fresh broccoli to obtain broccoli dry powder; (2) smashing new harvested mustard seeds, adding distilled water, and carrying out ultrasonic oscillation and suction filtration to obtain crude enzyme; (3) uniformly mixing the broccoli dry powder with the crude enzyme, carrying out enzymolysis and drying to obtain dry broccoli dry powder; (4) adding ethyl alcohol to the broccoli dry powder, carrying out ultrasonic oscillation extraction, sieving and suction filtration to obtain sulforaphane crude extract; (5) concentrating the sulforaphane crude extract to obtain sulforaphane aqueous solution; (6) diluting the sulforaphane aqueous solution to obtain the sulforaphane aqueous solution with the sulforaphane content of 3-5mg/mL; (7) adding the sulforaphane aqueous solution to the dry broccoli powder, and drying after uniform mixing to obtain redried broccoli powder; (8) smashing the redried broccoli powder for capsule filling to obtain the sulforaphane capsule with the sulforaphane content of 6-14mg/g. The preparation method is simple to operate and low in cost.