The invention discloses a preparation method of nano calcium carbonate used for a weather proofing silicone sealant, the proportion of lime milk aged for 36-72h is adjusted to 1.055-1.070, the temperature is controlled between 18 to 28 DEG C, lime kiln gas is introduced into for carbonation reaction, the lime kiln gas flow is controlled to 6-10m<3> / h, the carbon dioxide volume content is 25-40%, when the reaction solution pH is reduced to 7, the reaction solution is heated to 50 to 90 DEG C, 12-20g of fatty acid saponified solution and polyamide wax compounded surface treatment agent is added for coating, and after 60-120min of stirring, the product is obtained by completion of surface modification, dehydration, drying, crushing and grading. The nano calcium carbonate prepared by the method can significantly shorten surface drying time, reduce the curing silicone sealant modulus, and improve the filling silicone sealant thixotropy, processing and adhesion properties, and the process is easy and feasible, low in production cost, and good in economic benefit.