Abstract: Objective To explore the ultrasound characteristics of accessory cavitated uterine malformation (ACUM) and the causes of misdiagnosis, in order to strengthen the understanding of the disease and improve the diagnostic ability of radiologists. Methods We retrospectively collected clinical, pathological and imaging data of ACUM patients who were diagnosed after surgery at Peking Union Medical College Hospital from December 2013 to December 2023. Besides, we analyzed ultrasound features of ACUM and the cause of misdiagnosis. Results A total of 11 patients were included, with a median age of diagnosis of （27.1±7.4） years. There was a mean interval of （6.1 ±5.5 ） years between menarche and onset of dysmenorrhea. Preoperative ultrasound showed ACUM appeared as solitary lesions located under the insertion of the round ligament, with a median maximum diameter of （3.0±1.0） cm. Lesions were round or oval (90.9%, 10/11) and not connected to the uterine cavity (100%, 11/11) with clear boundaries (72.7%, 8/11), presenting as regular thick-walled cystic-solid structures. Cystic areas were observed within the mass (100%, 11/11) with a ground-glass-like appearance (90.9%, 10/11) and hyperechoic endometrial lining (90.9%,10/11). The surrounding area of the mass showed resembling the muscular layer and circular or semi-circular vascularity was detected around the mass. No adenomyosis or other uterine lesions were found. Analysis of misdiagnosis: Among the 11 ACUM cases, 6 were correctly diagnosed by the initial ultrasound examination, and 5 cases by MRI. 2 patients were misdiagnosed as residual horn uterus by MRI, despite ultrasound suggesting ACUM with visible bilateral uterine horns. Conclusion ACUM exhibits distinct ultrasound characteristics. When young women present with progressive dysmenorrhea and ultrasound reveals a regular cystic-solid mass with thick wall located under the round ligament insertion, ACUM should be considered.