Background It has been known that ABO blood groups are linked to the phenotypes of certain diseases; however, and the relationship between ABO blood groups and postoperative pain have not been extensively studied, especially in children. This study was to investigate whether there would be an association between the four major ABO blood groups and postoperative pain, as indicated by the differences in pain scores and rescue fentanyl requirements among blood groups in children after adenotonsillectomy. Methods A total of 124 children, aged 3–7 years, ASA I or II, and undergoing elective adenotonsillectomy were enrolled in the study. Postoperative pain was evaluated using the Children’s Hospital of Eastern Ontario Pain Scale (CHEOPS) and the rescue fentanyl requirement in post anesthesia care unit (PACU) was analyzed. Pediatric Anesthesia Emergence Delirium (PAED) score and the duration of PACU were recorded. The postoperative nausea and vomiting (PONV) within 24 h were documented. Results Among four blood type groups, no significant differences were observed regarding surgery time, and the gaps of fentanyl given at the anesthesia induction and the first rescue fentanyl injection in PACU. However, patients from AB and B blood groups had significantly higher pain score at initial CHEOPS assessment and consequently, higher consumption of rescue fentanyl during PACU stay. A significantly higher percentage of patients had received > 1 μg/kg rescue fentanyl. Higher PAED scores were also observed in AB and B blood groups. Conclusion Paediatric patients with AB and B blood type had higher postoperative CHEOPS pain score and required significantly more fentanyl for pain control than those with A and O blood type after T&A. The initial scores of PAED in patients with AB and B blood type were also higher than that in patients with A and O blood type.