The main diagnostic tools for congenital megacolon are rectal manometry, barium enema angiography and rectal pathology biopsy. Of course, clinical manifestations and physical examination are the most basic diagnostic basis. Especially, rectal examination is important for the diagnosis and exclusion of megacolon. Rectal finger examination can help determine the development and lesions of the rectum and anus in order to exclude defecation difficulties caused by abnormal development, malformation or acquired factors of the rectum and anus. Rectal finger examination can alert us to the suspicion of megacolon if a narrow ring is felt or if the stool ejects in a blast-like manner after finger withdrawal. Anorectal manometry is mainly to measure the relaxation reflex ability of the internal sphincter and the pressure of the anal canal. In normal people, the internal sphincter relaxes when defecating, but in patients with megacolon, the internal sphincter relaxation reflex cannot be produced due to ganglion cell deficiency. The anorectal manometry uses this principle to simulate whether the internal sphincter relaxation reflex can be produced during defecation to help diagnose megacolon, and this test has a certain false-negative and false-positive rate in the neonatal period, with increasing accuracy as the age increases, reaching more than 90% accuracy at 2 months of age. When congenital megacolon is suspected, an X-ray barium enema angiogram should be done mainly to help the doctor obtain information about the length of the diseased intestinal canal and the degree of secondary dilatation of the intestinal canal in the child, so that the doctor can decide on the treatment plan and surgical options. Nowadays, the amount of radiation from X-ray machines is relatively low and does not affect the child much. The barium is mainly injected into the colon through the anus, and is eliminated through bowel cleansing afterwards. As long as no intestinal perforation occurs causing peritonitis, there is no effect on the baby. It is worth noting that abdominal ultrasound cannot diagnose congenital megacolon in the current situation. If the baby’s constipation persists and cannot be improved by various conditioning and conservative treatment methods, barium enema angiography does not show obvious morphological changes in the intestine, and the results of anorectal manometry are ambiguous, a pathological biopsy should be considered at this time. There are two main methods of pathological biopsy, one is rectal mucosal biopsy and the other is rectal whole-layer biopsy. Among them, rectal mucosal biopsy can be done under anesthesia in the operating room or in the ward. The key is to obtain tissues large enough and pathologists with more diagnostic experience to have a better diagnostic rate, while rectal mucosal biopsy also has the risk of intestinal bleeding and intestinal perforation. If the rectal mucosal biopsy is also not diagnostic, or if the child needs a caesarean section for various reasons such as intestinal obstruction, a whole rectal biopsy can be done, which is the gold standard and the last resort for the diagnosis of megacolon.