In the head and neck region, small cell neuroendocrine carcinoma (SmNEC) is extraordinary infrequent. Collision malignancy is also a rare entity in the nasal cavity, with merely sporadic 6 case reports on primary collision tumor associated with neuroendocrine carcinoma. The development of a secondary SmNEC within the previous radiation field had uncommonly been described, and there was no report on secondary sinonasal collision carcinoma with SmNEC component as a side reaction of nasopharyngeal carcinoma (NPC) radiotherapy. In light of the rarity of these neoplasms, we presented a case of a sinonasal collision carcinoma of papillary squamous cell carcinoma (PSCC) and SmNEC after NPC radiotherapy. To our knowledge, it may be the first case of a secondary coexistence of two malignancies synchronously in the nasal cavity after NPC treatment. Recognizing this peculiar kind of collision tumor associated SmNEC could promote our understanding of this entity and hence propose optimal treatment strategies.