High-density shadow in the lung is an imaging description, not necessarily a precursor of lung cancer. It mainly depends on the comprehensive analysis result of CT report, especially the high-density shadow in the upper lobe of the lung is likely to be congenital or calcified shadow left by tuberculosis. Therefore, to determine whether it is a precursor of lung cancer, further pathological examination is needed. High-density shadow in the lung can be divided into patchy shadow and mass shadow. Patchy shadow is mostly caused by tuberculosis disease or lung inflammation, such as tuberculosis, calcified foci in the lung, granuloma, fibrosis, pneumonia, etc. After systematic anti-inflammatory and anti-tuberculosis treatment, the shadow can gradually decrease or disappear. If the high-density shadow of the lung is in the shape of a mass, if its shape is irregular or if there are burrs at the edge and the burrs are lobulated, then lung cancer is more likely. Overall, if the CT exam only detects a high-density shadow in the lung and there are no other specific symptoms, most of them are benign diseases. Therefore, it is important to pay more attention to lung maintenance in daily life, and it is especially important to quit smoking.