Radiation in radiotherapy mainly kills tumor cells in two ways, as follows: 1) direct action: after cancer cells are exposed to ionizing radiation, it directly damages the double-stranded DNA of cancer cells, which eventually leads to cell death and makes its cellular DNA unable to replicate; 2) indirect action: radiation acts on water molecules inside cancer cells and forms oxygen free radicals; oxygen free radicals are more damaging to cells, and through the indirect water molecules interact with each other to form and release a large number of free radicals, which can kill cancer cells. Ionizing radiation generally acts on cancer cells through both direct and indirect effects together, which eventually leads to apoptosis and necrosis of cancer cells. In addition to the killing effect of radiation on cancer cells, it also has corresponding damage to the normal tissues, organs and cells around the cancer cells. Therefore, during radiation therapy, great attention should be paid to protect normal tissues and control the dose of its radiation to normal tissues so that normal tissues are protected from ionizing damage.