Objectives Although the guidelines clearly recommend the fasting time of children before anesthesia, it is usually difficult to control. For pediatric patients, prolonged fasting time before surgery will lead to dehydration and hypoglycemia. Adenotonsillectomy is one of the most common operations in pediatric patients, but its complications are not rare. The purpose of this study is to analyze the relationship between preoperative fasting time and postoperative complications in children undergoing adenotonsillectomy. Methods The medical and surgical records of 480 pediatric patients who underwent adenotonsillectomy were analyzed retrospectively. They were divided into three groups, including adenoidectomy group, tonsillectomy group, and adenotonsillectomy group. Logistic regression analysis was used to analyze the effect of preoperative fasting time on postoperative complications and hospital stay in pediatric patients of the three groups. Results The postoperative bleeding rate in the adenoidectomy group (5.16%) was lower than tonsillectomy group and adenoidectomy group (P < .001). Logistic regression analysis showed that the fasting time was positively correlated with the vomiting and pain in adenoidectomy group, tonsillectomy group, and adenotonsillectomy group. And, the postoperative hospital stay was also positively correlated with fasting time in three groups. Conclusion The prolonged fasting time before otolaryngology surgery in children is related to the occurrence of postoperative complications like vomiting and pain, and also to the increase of postoperative hospital stay.