The invention belongs to the field of biological medicine, and particularly relates to a trigger type ultrasonic contrast agent and a preparation method thereof. The ultrasonic contrast agent comprises core-shell particles, and each core-shell particle comprises a shell and a core; each shell comprises a shell membrane formed by a film-forming material, gold nanoparticles are inlaid in each shellmembrane, and each core is perfluorooctylbromide. The trigger type ultrasonic contrast agent has stable characters under normal conditions, and can safely and efficiently undergo photo-induced phase transition under the laser action, thereby optimizing the enhanced ultrasonic imaging effect of the contrast agent in vivo. The ultrasonic contrast agent can be applied to the operation of accurately detecting local lymph node metastasis of tumors, and has great clinical significance for disease course staging, progress and prognosis and formulation of an effective treatment plan for tumor patients.