Introduction: Left atrial appendage aneurysm (LAAA) is an extremely rare cardiac abnormality with potentially life-threatening risks. Most LAAA were detected because of the symptoms triggered by it, while asymptomatic LAAA were detected incidentally. Once LAAA is diagnosed, early surgical intervention is generally recommended. Case presentation: We reported a case of an 18-year-old female with giant congenital LAAA without any symptoms or surgical history. In an incidental transthoracic echocardiography (TTE) examination, a saccular structure adjacent to her left rear of the heart was encountered. Further CT and MRI examination established the diagnosis of LAAA. The LAAA was corrected via open heart surgery with cardiopulmonary bypass, and this patient recovered well and discharged uneventfully. Conclusion: TTE is the primary tool to diagnose LAAA. Transesophageal echocardiography (TEE), CT and MRI can be used for more accurate diagnosis. Early operation is a generally recommended and widely used intervention to prevent adverse events.