I. Overview Pain is one of the most common symptoms of cancer patients, which seriously affects the quality of life of cancer patients. The incidence of pain in first diagnosed cancer patients is about 25%; the incidence of pain in advanced cancer patients is about 60%-80%, and 1/3 of them have severe pain. If cancer pain (hereinafter referred to as cancer pain) is not relieved, patients will feel extremely uncomfortable, which may cause or aggravate anxiety, depression, fatigue, insomnia, loss of appetite and other symptoms, and seriously affect patients’ daily activities, self-care ability, interaction ability and overall quality of life. In order to further standardize cancer pain diagnosis and treatment in China, improve the standardized diagnosis and treatment system of major diseases, improve the level of cancer pain diagnosis and treatment in medical institutions, improve the quality of life of cancer patients, and guarantee medical quality and medical safety, this specification is formulated. Cancer pain etiology, mechanism and classification Cancer pain etiology: 1. Tumor-related pain: caused by direct invasion and compression of local tissues by tumor, and tumor metastasis involving bone and other tissues. 2. 2.Anti-tumor therapy-related pain: commonly caused by surgery, traumatic examination operations, radiation therapy, and cytotoxic chemotherapy drugs. 3. Non-tumor-related pain: including pain caused by other comorbidities, complications and other non-tumor factors. Cancer pain mechanism and classification: 1. Pain is mainly divided into two types according to pathophysiological mechanism: injury-receptive pain and neuropathic pain. 2. Pain is divided into acute pain and chronic pain according to the duration of onset. Most cancer pain is chronic pain. Compared with acute pain, chronic pain lasts longer, the etiology is not clear, the degree of pain and the degree of tissue damage can be separated, and it can be accompanied by nociceptive hypersensitivity, abnormal pain, and poor efficacy of conventional analgesic treatment. The mechanisms of chronic pain and acute pain have both commonalities and differences. In addition to the basic conduction modulation process of injury-receptive pain, chronic pain can also show neuropathic pain mechanisms different from acute pain, such as overexcitation of injury receptors, ectopic electrical activity of damaged nerves, over-sensitivity of central mechanisms of nociceptive transmission, abnormal expression of ion channels and receptors, and central nervous system remodeling. Cancer pain assessment is a prerequisite for reasonable and effective pain relief treatment. Cancer pain assessment should follow the principles of “routine, quantitative, comprehensive and dynamic” assessment.