Objective To investigate clinical features of accidental parathyroid adenoma (APTA) and to explore diagnosis and treatment strategies of APTA. Methods From February 2009 to December 2016, the patients who would receive the thyroid surgery and were accidentally found the parathyroid adenoma by preoperative examination in the Department of Thyroid & Parathyroid Surgery, West China Hospital of Sichuan University were enrolled in the research. The clinical characteristics, surgical procedure, results of postoperative follow-up were analyzed retrospectively, and which were compared between the patients with APTA and the other patients diagnosed as primary parathyroid adenoma or received thyroid surgery (1 : 4 chosen randomly) in the same period. Results From February 2009 to December 2016, the patients who treated with thyroid surgery and were diagnosed as the primary parathyroid adenoma in our center were 5 881 and 251 respectively. Twenty-six patients with APTA were found in this research. The incidence rate of APTA was 0.44% (26/5 881), accounted for 10.4% (26/251) of the primary parathyroid adenoma. The positive rates of the ultrasound and the parathyroid scintigraphy were 69.2% (18/26) and 72.7% (8/11), respectively. The abnormal rate of the bone mineral density examination was 85.7% (6/7). The preoperative PTH was (38.17±40.69) pmol/L (3.40–181.20 pmol/L), and the serum calcium was (2.73±0.27) mmol/L (2.22–3.23 mmol/L). The number of detected parathyroid adenoma was 29, which were 55.2% (16/29) in the right-lower, 6.9% (2/29) in the right-upper, 27.6% (8/29) in the left-lower, and 10.3% (3/29) in the left-upper location. The rate of single parathyroid adenoma was 88.5% (23/26) and the maximum diameter of parathyroid adenoma was (21.72±9.65) mm. There was 13 cases (44.8%) of the A1 type and 16 cases (55.2%) of the B1 type in these 29 parathyroid adenomas. The rates of the recurrence, postoperative transient hypoparathyroidism, and permanent hypoparathyroidism were 7.7% (2/26), 30.8% (8/26), and 3.8% (1/26), respectively. Additionally, the preoperative PTH and serum calcium levels of the patients with APTA were significantly lower as compared with the primary parathyroid adenoma (P<0.001,P<0.001), which were significantly higher as compared with those of the patients received thyroid surgery without APTA in the same period (P=0.001, P<0.001). Conclusions APTA is a specific type of asymptomatic primary hyperparathyroidism. Examinations for PTH and serum calcium levels before thyroid surgery are important for finding APTA. For the patients with APTA, it is safe and effective to carry out exploratory parathyroidectomy with thyroid surgery at the same time.