Pain will bring early and long-term adverse reactions to infants. The investigators need to pay attention to whether there is pain in infants after surgery. Since infants cannot self-report pain,The investigators need to use appropriate pain assessment scale to evaluate the pain of these infants, so as to understand the status of postoperative pain in children. The result of pain score not only enables investigators to understand the pain status of children, but also helps investigators to give corresponding intervention and treatment according to the pain degree of children. Postoperative pain management is one of the core contents of ERAS. Effective pain management is beneficial to the early postoperative recovery of infants and reduces the adverse reactions caused by pain. Sveral studies have confirmed that the combination of acetaminophen and opioids could reduce the use of opioids after surgery. But even if opioid use is reduced, it still causes many side effects for children. This study evaluated the safety and efficacy of acetaminophen alone for postoperative analgesia in infants.

Inclusion Criteria:

• Informed consent was obtained from parents of children;
• Infants aged 0-12 months;
• Children who underwent abdominal and perineal surgery in the Department of Neonatal Surgery, Children's Hospital Affiliated to Nanjing Medical University and returned to the unaccompanied ward after removal of tracheal intubation.

Exclusion Criteria:

• No informed consent was obtained from parents;
• known allergy or intolerance to acetaminophen;
• Children with liver function impairment (alanine aminotransferase/aspartate aminotransferase (ALAT/ASAT) more than three times the reference value;
• Children with renal insufficiency (urea and creatinine values exceeding the upper limit of reference values);