Introduction An anomaly of the duodenal shape is one of the rare congenital anomalies and remains poorly known in many previous studies and the literature. The duodenum is formed by developing the terminal foregut and proximal midgut through four stages in the embryonic period. According to the anatomy, the duodenum is typically described as C–shaped, U–shaped, or even horseshoe-shaped. Presentation of case The patient was hospitalized for abdominal pain and jaundice and diagnosed with ampullary carcinoma. During surgery, we incidentally discovered that the duodenum was not a C-shape. The first part of the duodenum and proximal half of the second part descended the head of the pancreas. However, the distal half of the second part bent to the right and ascended upwards to the upper-right margin of the pancreatic head. After that, the third part ran slantingly downward to the left and posterior of the pancreas and portal vein. Discussion During the fifth week, the ventral pancreatic bud moves around the duodenum's posterior side and unites the dorsal pancreatic bud at the sixth week. The place of the distal half of D2 migrated abnormally after ventral pancreatic bud rotation finished. The rapid and premature elongation of the proximal midgut, the influence of a very fast enlarged liver, or the early return of the umbilical loop combine with insufficiently developed abdominal space. These reasons may have led to the abnormal folding of the D2 position. Conclusion Knowledge about this anomaly helps clinicians know the duodenal-anatomical abnormalities.