Plunging ranula is rare and manifests as the submandibular or submental cystic mass, without intra-oral abnormality. It usually causes misdiagnosis and malpractice. This article reported a case of plunging ranula that appeared as a massive mass located behind the left ear. A 6-year-old child presented with recurrent left retroauricular swelling over six months without obvious inducement. The mass showed repeatedly swelling and persistent skin ulcers. Physical examinations described a fluctuant, nontender mass behind the left ear whose size was about 5 cm×5 cm×3 cm. The skin upon the surface of the mass was thin and red, and overflowed yellow slime-like contents. Computed tomography (CT) showed an extensive cystic lesion in the left neck. After local incision and drainage the mass was shrink. With the primary clinical diagnosis of branchial cyst, the patient underwent mass resection through postauricular incision. During the operation, the fistula was traced to the area around the sublingual gland, and the postoperative pathology report demonstrated cystic spaces occurring in soft tissue without lymphoid tissue. One month postoperatively, the patient presented the “egg-white”, wire-drawing transparent viscous fluid outflowing from the left external auditory canal, indicating that the fluid in the external auditory canal originated from the sublingual gland and the disease was the plunging ranula presenting as a giant left retroauricular mass. We readmitted the patient to the hospital and the ipsilateral sublingual gland was completely removed in the mouth under general anesthesia. No clinical evidence of recurrence was found at the end of 9 months follow-up. So far, there is no literature reporting plunging ranula behind ear. This case report describes the clinical manifestation, diagnosis, and differential diagnosis and treatment of a case of plunging ranula, and reveals that, other than the submandibular or submental, plunging ranula also could be found in retroauricular region. The plunging ranula is difficult to fully confirm through a clinical character, and usually easy to be misdiagnosed as other tumors, like lymphatic malformation, venous malformations, etc. Radiographic examination and/or puncture fluid observation are required for identification. “Egg-white”, wire-drawing mucous secretion is the most important characteristic in clinical diagnosis of sublingual gland cyst. However, if sublingual gland resection, i.e. removal of the source of cyst fluid is performed, a good prognosis can be achieved.