The difference between precancerous lesions of lung cancer and carcinoma in situ exists in the depth of infiltration, the presence of cancer cells, and treatment methods. 1. Pre-cancerous lesion: there is no cancerous lesion in the lungs, or even only highly atypical hyperplasia, which has the possibility of cancerous lesion but without the presence of cancer cells. Most of them are hard to be detected clinically, and their treatment principle is early detection, early diagnosis and early treatment to prevent cancer from occurring. 2. Carcinoma in situ: Lung carcinoma in situ belongs to stage 0, and there are cancerous changes, i.e. there are cancer cells. However, the cancer has not broken through the basal layer and is only confined to the mucous membrane layer, and there is often no lymphatic or distant metastasis. Surgery is the main treatment. It is recommended that the patient should consult a doctor and get a clear diagnosis under the guidance of a professional doctor.