Effective postoperative pain control is essential for patients and inadequate postoperative pain relief can cause mental and psychological sufferings. Despite the growing concern on postoperative pain management, acute postoperative pain is still poorly managed. Though numerous clinical practice guidelines for postoperative pain management have been published throughout the last decades, inadequate pain relief remain a big health care issue. Sufentanil has been used as satisfied pain control drug because of its strong potency of analgesia for a long while. But its use in patient controlled intravenous analgesia (PCIA) has not been clarified. And the dilemma of safety concern and insufficient dosage of analgesic is a common problem. Thus the investigators design this prospective randomized controlled double blinded trial to observe the efficacy and the optimal dose of sufentanil in PCIA in patients underwent moderate surgery for the purpose of providing reference in clinical practice.