Colorectal cancer (CRC) in pregnancy is rare. The clinical manifestations of CRC during pregnancy are not specific, and diagnosis and treatment pose a significant challenge. Patients are often found to have advanced tumors, and have a poor prognosis. In this case study, the patient was 36 years old, and had no obvious clinical manifestations in the first and second trimesters. Since gestational week 38, she experienced left upper abdominal pain and constipation, with no nausea or vomiting. Imaging examinations revealed malignant tumors of the sigmoid colon (colon cancer was highly suspected), multiple liver metastases, omental metastases, and multiple swollen lymph nodes in the abdominal cavity. After discussion, the patient received lower cesarean section. A large amount of hematochezia with a volume of approximately 1,000 mL occurred 8 days after the operation. In the emergency department, superior and inferior mesenteric angiography was performed, and the inferior mesenteric artery was continuously pumped with pituitrin to stop bleeding. After the condition was stabilized, she underwent surgery for radical sigmoid resection and colon single-cavity fistula. During the operation, light bloody ascites were observed inside the abdominal pelvis, approximately 500 mL, with multiple touchable stiff metastatic nodules in the liver. At the side of the sigmoid mesocolon, a giant tumor of approximately 12 cm diameter was found, continued by the intestinal wall, and the sigmoid mesocolon was almost occupied by the tumor. The mesentery was hard to recognize, with possible movement of the tumor, and no sign of infiltration into the abdominal pelvic wall or adnexa. The surgical pathological stage was Dukes' stage D, which was an advanced tumor. Postoperative radiotherapy and chemotherapy were not performed, and the patient died of systemic multiple organ failure 32 days after colon cancer surgery (postpartum day 43). So for patients with digestive system symptoms during pregnancy, CRC should be considered in differential diagnosis. Auxiliary examinations should be actively carried out to strive early diagnosis and treatment to improve patient prognosis.