Objective: To observe the cytological pathological features of secondary thyroid neoplasm (STN) by ultrasound-guided fine-needle aspiration（FNA）. Methods: A total of 5023 cases of thyroid malignancy diagnosed by FNA cytology in The First Affiliated Hospital of Air Force Medical University from January 2011 to May 2021 were collected. Hematoxylin-eosin (HE) staining, Pap staining and cellular immunohistochemistry (IHC) staining were performed on traditional smears, liquid-based preparation and cell wax blocks made from FNA samples of all these STNs respectively. Using medullary thyroid carcinoma smear as a control, two senior cytopathologists analyzed the cytopathological and cellular immunohistochemical features of STN. Based on clinical practice, the key points of cytopathological differentiation between STN and medullary thyroid carcinoma were summarized. Result: Among 5023 cases of thyroid malignancy, there were 8 cases of STN (0.16%), 33 cases of medullary thyroid carcinoma (0.66%), 4955 cases of papillary thyroid carcinoma (98%), 16 cases of lymphoma (0.32%), 7 cases of squamous cell carcinoma (0.14%), and 2 cases each of anaplastic carcinoma and poorly differentiated carcinoma (0.04%). The 33 cases of medullary carcinoma tumor cells were diverse in morphology, including plasmacytoid cells, epithelioid cells, and fusiform cells, with a small number of bizarre giant cells and amyloid in the background. None of the 8 STNs showed typical nuclear features of thyroid papillary carcinoma， three-dimensional papillary and adenoid structures were seen in 1 case of metastatic lung adenocarcinoma， clusters or scattered heterotypic squamous cells were seen in 4/4 cases of metastatic squamous cell carcinoma, combined with their respective characteristic immunophenotypes, the diagnosis can be confirmed. The cytological morphological characteristics of 2 metastatic breast cancers and 1 metastatic small cell lung carcinoma overlapped with medullary thyroid carcinoma. The cells were all small and hyperchromatic nuclei, which required the detection of immunohistochemical markers including estrogen receptor (ER), Progesterone receptor（PR）, Cytokeratin7 (CK7), GATA3, Synaptophysin (Syn), Neural cell adhesion molecule (CD56), Chromogranin A (CgA), Calcitonin (CT), Thyroglobulin(TG), Thyroid transcription factor-1(TTF-1), Paired box gene 8 (PAX-8), and identification were made in the combination with medical history. Conclusions: STN accounts for 0.16% of thyroid malignancies and is relatively rare. In cases with myeloid cancer cell morphology, attention should be paid to the possibility of STN, and differential diagnosis should be made in combination with immunohistochemistry and medical history.