The invention belongs to the technical field of medicine, and relates to application of indolepopionic acid to preparation of drugs for preventing and / or treating tumor radiotherapy side effect. Through research, the fact that indolepopionic acid can obviously improve intestinal injury of mice, caused by gama ray irradiation, is found, the improvement shows that the survival rate of laboratory mice is obviously improved, the weight is risen again, the intestinal inflammation level is obviously reduced, and paradoxical reaction of the laboratory mice does not occur, the fact that indolepopionicacid can be effectively applied to the prevention and / or treatment of tumor radiotherapy side effect is shown, and the indolepopionic acid is low in toxic and side effect, and effective before and after being taken, and has a wide application potential.