Objective To investigate the ultrasonographic features features of congenital gastric antrum septum in children before and after oral administration of contrast agent, and to analyze the causes of missed diagnosis and misdiagnosis. Methods Children diagnosed with congenital antral septum by surgery and pathological examination in our hospital between January 2012 and June 2020 were collected. Their clinical characteristics and the results of ultrasound, upper gastrointestinal contrast, gastroscopy and histopathological examination were retrospectively analyzed. The sonographic characteristics of ultrasound diagnosis were summarized, the causes of missed diagnosis and misdiagnosis were also analyzed. Results A total of 24 children were included, including 20 male and 4 female; The main clinical manifestation was recurrent non-biliary vomiting (23 cases, 95.83%). 8 cases (33.33%) of gastric antral septum were accurately diagnosed by ultrasonography, 15 cases (62.50%) were missed and 1 case (4.17%) was misdiagnosed; Among them, only 2 cases were accurately diagnosed without oral contrast agent, and 8 cases (including those 2 cases already diagnosed) were accurately diagnosed after taking contrast agent. Under contrast-free ultrasound, the gastric antrum septum presented as a narrow strip of low echo perpendicular to the anterior and posterior walls of gastric antrum, with local narrowing of gastric antrum; Under contrast-enhanced ultrasound, the septum was a narrow strip with "high-low -high" three line echo structure, which was 0.8-1.2 cm in length and 0.18-0.6 cm in thickness; The diaphragms in different positions presented different shapes. Diaphragms in gastric antrum (6 cases) presented as strip structure vertical to the long axis of the gastric antrum. A central or eccentric apertures, whose size ranged from 0.13 to 0.5 cm, was observed in the diaphragm (4 cases). Diaphragms in the pylorus (2 cases) looked like redundantly folded mucosa, which was edematous and thickened and with fixed shape, forming the "Nipple sign". Ultrasound also identified some indirect signs of gastric antrum septum, including gastric retention (18 cases, 75%), narrowed pyloric opening, and thickening of pyloric muscle layer (3 cases, 37.50% ). Conclusion The identification of hypoechoic strip structure, local narrowing of gastric antrum, limited opening of pylorus and edema and thickening of pylorus myometrium would be very helpful in the initial screening of gastric antrum septum by routine ultrasound examination. Further detection with contrast-enhanced ultrasound can show the structure and apertures of the diaphragm, which is critical to the confirmation of the diaphragm and the judgment of the degree of gastric antral obstruction, and finally greatly promotes the early diagnosis and early intervention of this pediatric disease.