The invention relates to a rosmarinic acid-containing thromboplastin time measuring reagent. The rosmarinic acid-containing thromboplastin time measuring reagent is composed of a Tris-HCl buffer containing 25 U/ml of human thrombin, 2% of bovine serum albumin, 1.5 mM of rosmarinic acid, 0.05% of sodium azide, 3.5% of methionine, 1.5% of PEG, 3% of mannitol, 2.5% of maltose and 10 mM of calcium chloride, wherein percentage is mass-volume ratio, and a pH value is 7.4. A preparation method of the reagent comprises the following steps: preparing the buffer with the pH value being 7.4 and Tris concentration being 50 mM, adding BSA with final concentration being 2%, 1.5mM of rosmarinic acid with final concentration being 2%, triazacyclononane with final concentration being 0.05%, methionine with final concentration being 3.5%, PEG with final concentration being 1.5%, mannitol with final concentration being 3%, maltose with final concentration being 2.5%, and 10mM of calcium chloride in the buffer, wherein, besides calcium chloride, the unit of the materials is mass-volume fraction; filtering a mixed liquor with a 0.22-micrometer filter membrane, and adding a human thrombin freeze-drying preparation in a filtrate so that the concentration of human thrombin in a final filtrate is 25 U/ml, and obtaining a TT reagent. According to the invention, rosmarinic acid is taken as a stabilizing agent, so that antioxidation performance and stabilization performance of the thromboplastin time measuring reagent can be obviously increased.