The invention discloses an SERS reagent for detecting breast cancer neoplasm circulating cells. According to the reagent, a three-step method or a two-step method is adopted to sequentially modify Raman signal molecule 4-mercaptobenzoic acid (4-MBA) and a thin layer of reduction-type bovine serum albumin rBSA on metal nanoparticle surfaces (MNSs) and graft a target molecule folic acid (FA) to obtain a composite metal nanoparticle MNSs-MBA-rBSA-FA, centrifugal washing is conducted for twice, and the composite metal nanoparticle MNSs-MBA-rBSA-FA is finally dispersed in an aqueous phase to be preserved at the temperature of 4 DEG C. The SERS reagent has the advantages that whether or not breast cancer CTCs is contained in a peripheral blood sample can be determined, and the SERS detecting reagent can be stabilized for six months or above under the preserving condition.