Concurrent radiotherapy and chemotherapy is the main treatment for locally advanced non-small cell lung cancer. Acute Radiation Pneumonitis (ARP) is one of the most important complications of radiotherapy for lung cancer and is difficult to treat and causes high mortality rates. In this study, we reported a patient with grade IV ARP who underwent 2 months of radiotherapy. This patient showed significantly different clinical features from common ARP, he had early symptoms of illness without evident signs and imaging manifestations, rapid short-term illness progression, wide range of lesions, and bystander effect. We described our experience in the diagnosis and treatment of ARP, and discussed the importance of early prevention of ARP during radiotherapy.