Abstract: Immune checkpoint inhibitors (ICIs)-based tumor immunotherapy has changed the traditional cancer treatment. However, ICI treatment benefits small percentage of patients in most types of cancer (10%-30%), and is basically ineffective in some cancers (such as pancreatic cancer and glioma). Combining ICIs with existing and potential therapies to overcome tumor innate and acquired resistance is of great significance for improving the treatment efficacy, increasing the durability of the therapeutic effect and prolonging patients' survival. Radiotherapy can not only kill tumor cells, but also cause the release of pro-inflammatory molecules and immune cell infiltration in tumors. In addition, radiotherapy can induce micronuclei in tumor cells, thereby activating cytosolic DNA/RNA sensors, the most important of which is the cyclic GMP-AMP synthase (cGAS)-STING pathway. Radiotherapy can also regulate immune surveillance through the expression of tumor neoantigens. In this review, we will discuss in depth the immunomodulatory effect of radiotherapy on the tumor microenvironment and its combination with ICI as a potential cancer treatment, and focus on the effects of radiotherapy on non-tumor cells in the tumor microenvironment, including dendritic cells, T cell infiltration, as well as myeloid-derived suppressor cells.