The invention discloses a nipple sealing agent for preventing mastitis of cows in a dry period and a preparation method thereof, and belongs to the field of veterinary medicines. The nipple sealing agent consists of the following components in percentage by weight: 40-75wt% of a non-toxic metal compound, 2-10wt% of geniposide and 10-50wt% of a gel matrix, wherein the gel matrix consists of the following components in percentage by weight: 70-90wt% of a dispersion medium, 5-12wt% of gels, 2-5wt% of a suspending agent, 0.1-0.3wt% of an antioxidant and 0.04-0.15wt% of a preservative. According to the nipple sealing agent disclosed by the invention, effective components namely geniposide and the non-toxic metal compound are matched with each other to ensure that the nipple sealing agent has high cure rate on mastitis of the cows in the dry period and can be used for effectively reliving the inflammatory reaction of clinical mastitis.