Trace elements are important and indispensable nutrients in human metabolism and their intake is relatively small but they are essential for maintaining normal physiological functions of the human body. Many patients receiving nutritional therapy have varying degrees of trace element deficiency due to various reasons leading to disease progression and affecting clinical outcomes. Cancer is one of the leading causes of death worldwide and poses a major threat to human health. In recent years the role of selenium as an essential trace element in the prevention and treatment of cancer has attracted much attention. Studies have shown that adequate selenium intake can reduce the risk of some types of cancer and has shown significant potential in improving the efficacy of chemotherapy and radiotherapy and reducing the side effects of treatment. Selenium plays an important role in anti-tumor processes through its antioxidant anti - inflammatory and immunomodulatory properties. The specific mechanisms include enhancing the activities of antioxidant enzymes inducing apoptosis inhibiting tumor angiogenesis and regulating the immune system. Compared with traditional anticancer drugs selenium has significant advantages in multi - target action low toxicity and side effects and cost - effectiveness. In addition selenium supplementation has shown promising effects in reducing toxic reactions caused by chemotherapy and radiotherapy. This article reviews the application progress of selenium in the prevention and treatment of tumors and analyzes its auxiliary role in chemotherapy radiotherapy and immunotherapy in order to provide new ideas for clinical practice and improve the treatment effect and quality of life of cancer patients.