The invention relates to a dual-component sulfhydryl-terminated polyurethane mildew-proof sealant and a preparation method thereof, wherein, the sealant consists of component A and component B, wherein, according to the parts by weight, the component A comprises 80-120 parts of sulfhydryl-terminated polyurethane liquid rubber, 20-200 parts of filling material, 10-100 parts of plasticizer, 0.5-10 parts of tackifier, 0.1-5 parts of mould inhibitor, 0-5 parts of vulcanization regulator, and 0-5 parts of antioxidant; the component B comprises 80-120 parts of vulcanizing agent, 20-150 parts of plasticizer, 20-200 parts of filling material, and 0-5 parts of vulcanization regulator; and the proportion of component A and component B is 10:0.5-2. The mildew-proof sealant has the tensile strength of 5.0MPa, breaking elongation rate of 1020 percent, the bonding strength with the concrete of 2.5MPa, and the mildew-proof level of being less than or equal to 1; the sealant can be used for the fields such as electrons, mechanicals, and the like, can meet the sealing and mildew-proof requirement under the conditions of long-term water immersion, damp-heat or other conditions where mildews are easy to breed, has no exhaust of three wastes and is friendly to environment.