Patient-controlled analgesia is a method of administering analgesics in small doses that is controlled by the patient. The minimum effective analgesic dose and the minimum effective blood concentration are very different for different individuals in different conditions, and there is a double risk of underdose and overdose with conventional doses of analgesic drugs. PCA treatment can maintain the blood concentration close to the minimum effective concentration. When the analgesia of patients with advanced cancer pain is inadequate or impossible to be administered via intestinal route or when they cannot tolerate the intractable side effects of intestinal drugs, PCA can be administered by subcutaneous, intravenous or epidural routes to achieve a stable and satisfactory level of analgesia, and at the same time, it can satisfy the patient’s need to control the instantaneous onset of pain and allow the patient to increase or decrease the dose of analgesic drugs according to his own needs. It can be used as a “three-step” supplemental treatment for cancer pain.