We know that after surgery patients suffer from varying degrees of pain, and this is often the reason why many patients are fearful of surgery. So what causes pain after surgery? Surgical trauma is an injurious stimulus that activates peripheral injury receptors (injury receptors are tiny sensory nerve endings that transmit pain signals). Local tissue damage can cause the body to become more responsive to the stimulus (nociceptive hypersensitivity), or it can cause the production and aggregation of a number of pain-causing substances, which cause pain sensations both locally in the injured tissue and around the injured tissue (in areas where there is no damage). When the injury receptors in the viscera are activated, smooth muscle spasm, ischemia, and inflammation can occur, and may also cause entrapment pain in the skin. Therefore, the pain that a patient will experience after surgery can be pain at the incision site (somatic pain) or both visceral pain. In general, somatic pain is somewhat limited and manifests as sharp pain; whereas visceral pain is poorly localized and more diffuse. The injurious stimuli and pain of surgery can induce a neuroendocrine stress response in patients, which results in an over-metabolic state of sodium retention and elevated blood glucose, which can hinder the patient’s postoperative recovery to a certain extent. Neuroendocrine stress is also a major factor in postoperative hypercoagulability. Hypercoagulability can cause deep vein thrombosis, myocardial ischemia, and vascular graft failure. Uncontrolled postoperative pain can also excite the sympathetic nerves and impair recovery of gastrointestinal function. In the case of upper abdominal and thoracic surgeries, pain may interfere with the patient’s ability to take deep breaths and cough up sputum in the postoperative period, leading to pulmonary complications. In addition, acute pain that is not well controlled may turn into chronic pain.