Primary hyperparathyroidism (PHPT) is a disorder of one or more of the parathyroid glands, which results in hypercalcemia. Hyperparathyroid crisis is a rare condition that sometimes occurs when people with PHPT experience another illness, like vomiting or diarrhea, which causes excessive fluid loss or severely limits the amount of fluid they can consume. Parathyroid adenoma and hyperplasia are the most common causes of PHPT. In clinical practice, functional parathyroid cysts are a very rare cause of PHPT. We report a rare case of a functional parathyroid cyst as a cause of hyperparathyroidism or even hyperparathyroid crisis. After surgery, the patient's blood calcium and PTH level returned to normal ranges. Histological examination showed in fibrous cystic wall, some of the inner wall of the capsule were covered with a single layer of flat epithelium, some were not covered with cells, and a small cluster of parathyroid tissue was extruded and deformed in the wall of the capsule.