Background: False aneurysm of ascending aorta is a rare and life-threatening disease. Most of them occur after cardiovascular surgery and have obvious clinical symptoms. Case presentation: A very healthy adult male was admitted to hospital due to mediastinal space occupying during routine physical examination. The patient had no history of surgery and no clinical symptoms. He was diagnosed with a pseudoaneurysm of the ascending aorta and underwent internal suture of the aneurysm with peripheral cardiopulmonary bypass. We also found that there was a kind of circular weak part in front of the ascending aorta, with a diameter of about 2cm. We don't know if he has immune related or unknown diseases related to the middle layer of the aorta. Finally, the patient recovered well. Conclusions: This case report can improve the awareness of the rare diseases of pseudoaneurysm of ascending aorta, and the diagnosis of ascending aortic pseudoaneurysm should not be ignored in patients with mediastinal space occupying lesions found in routine physical examination.