Appendiceal mucous cysts are rare, and about 0.43% of surgically resected appendix specimens are of this type. The mature cells of appendix are mainly mucus cells, when the mucus cells are still functional, the appendix becomes obstructed and the secreted mucus exists in the lumen, making the appendix look like a cyst, when the pressure in the appendix lumen increases to a certain degree, the mucosa loses its function, the epithelial cells become flattened and no longer secrete, so the size of appendiceal mucus cyst usually does not exceed 3.0 cm × 8.0 cm. The incidence of appendiceal mucus cyst is low, the onset is slow, the abdominal pain is vague, and in the absence of acute infection, the symptoms and signs resemble chronic appendicitis, so it is often difficult to diagnose, but when the volume is large, an intact oval mass without adhesions around it can be found on physical examination. Moreover, appendiceal mucous cysts come from appendiceal lesions and are easily misdiagnosed as common appendiceal abscesses, with a misdiagnosis rate of 94.6%, as also seen in this case on CT scan. However, CT and ultrasound are still important tools for preoperative differentiation of appendiceal mucous cysts from other lesions. About 10% of appendiceal mucinous cyst cases are malignant type, which is a true tumor. The cyst may rupture and implant on the peritoneum to form a peritoneal pseudomucinous tumor. It is easy to recur after surgery and prone to malignant behaviors such as peritoneal implantation. Lymphatic and hematogenous metastases usually do not occur, and most of them die due to intestinal obstruction or renal failure. Therefore, special attention should be paid to protection during surgery, and the appendiceal cyst should be removed without rupture. If the cyst is found to rupture intraoperatively, in addition to removing the appendiceal mucinous cyst, the abdominal cavity must be thoroughly cleaned, and fluorouracil can be injected intraperitoneally to prevent the occurrence of peritoneal pseudomucinous tumor.