The work is to study radiotherapy-induced clinical symptoms of endoscopic findings, pathological changes and evaluation of esophagitis in the thorax. The clinical symptoms of 66 patients under radiotherapy were evaluated with the accumulated dose, endoscopic findings of esophagus and pathological changes. Typical symptoms in 2—3 weeks included moderate dysphagia or odynophagia and requiring narcotic analgesics with 20—30Gy of the accumulated dose. Endoscopic findings of esophagus were with erosion of mucosa or without sloughing, pathological changes of esophagus were the infiltration of inflammatory cells in squamous epithelium, disintegration of basic layer, deeply stained necrosis of cells. The grade≥2 acute esophagitis using RTOG criteria was observed 21.2%, and the grade 3 is 3.0%. The grade 2 and 3 acute esophagitis using Kuwaht’s criteria was observed 38.5%. The diagnosis of acute esophagitis should include history of exposure to radiation, with doses exceeding the dose threshold, and typical clinical characteristics. Endoscopic examination is very useful for diagnosis, since such examination can provide accurate information. RTOG scores correlate closely with endoscopic grading, and is convenient and precise. Pathology can distinguish between benign and malignant changes of ulcer.