Abstract: Radiotherapy is an important treatment method for malignant tumors. Radiation resistance is the main obstacle to the therapeutic effect of radiotherapy. Cellular metabolic reprogramming is one of the main features of cancer, and it may have an important effect on the therapeutic effect of radiotherapy. Glutamine is closely related to tumor cell biosynthesis and growth. It affects radiotherapy sensitivity by producing antioxidants through decomposition. In addition, the expression patterns and functions of two isoenzymes of glutamine, namely, glutaminase (GLS) and glutaminase 2 (GLS2), are different and have an important influence on the sensitivity of radiotherapy. The utilization of glutamine metabolism in the tumor microenvironment has great research value to improve the efficacy of radiotherapy. This review describes the metabolic characteristics of glutamine in malignant tumors and the sensitization effect of glutamine inhibitors on the efficacy of radiotherapy.