Dark skin on the back is not necessarily a sign of lung cancer, but lung cancer can cause dark skin on the back, especially for patients given systemic chemotherapy and local radiotherapy, which can lead to skin damage and skin pigmentation, resulting in dark skin. The darkening of skin also requires professional consultation with dermatologists to exclude other skin diseases to avoid misdiagnosis as lung cancer. The diagnosis of lung cancer needs to be based on chest imaging, high-resolution CT of chest and enhanced CT of chest. If CT examination suggests occupancy, short burr, lobar shape in lung, as well as obstructive pneumonia, pulmonary atelectasis, pleural effusion, etc. appearing in central lung cancer, especially histopathological examination can confirm whether it is small cell lung cancer or non-small cell lung cancer, once lung cancer is diagnosed if skin darkening occurs again, the relationship between the two needs to be considered comprehensively. In conclusion, if local radiotherapy or systemic chemotherapy is given after lung cancer diagnosis, radiation dermatitis and drug-related skin damage need to be professionally evaluated by a dermatologist to see if it is caused by lung cancer.