The invention discloses a preparation method of a T1/T2 dual relaxation platinum-iron oxide-gold nanoparticle. CXCR4-targeted polypeptide is utilized to modify T1/T2 dual relaxation platinum-iron oxide-gold coupled nanoparticle. The Pt-Fe3O4-Au nanoparticle prepared by the method provided by the invention can realize oriented synthesis of an isomer by controlling the mole ratio of oleic acid to pentacarbonyl iron, can improve the stability of a nanoparticle double-dumbbell structure, and also achieves a stable T1/T2 dual enhanced mode, the CXCR4-Pt-Fe3O4-Au heterotrimer nanoparticle prepared on the basis has excellent magnetic properties, good biocompatibility and high targeting properties to triple-negative breast cancer, provides a new means for early diagnosis of triple-negative breast cancer, and can be used for preparation of a triple-negative breast cancer targeted contrast agent.