Carcinoma in situ is an early-stage lesion, and its size needs to be determined according to the degree of infiltration and location of invasion, and cannot be generalized, if it is confined to the epithelial cells, it is usually <1cm. Carcinoma in situ is a kind of early non-invasive cancer, which is stage 0 according to pathological staging, confined to the epithelial layer without infiltration and metastasis. Generally speaking, there is no obvious mass in naked-eye examination, or a few of them can be manifested as localized vesiculation, roughness and inequality, which can be seen in CT examination, and the size of carcinoma in situ, if confined to epithelium, is generally less than 1cm. The treatment of carcinoma in situ needs to be combined with different types of diseases and invasive location to choose suitable treatment. If treated at early stage, the prognosis is better and most of them can be cured. One should pay attention to regular checkups, develop good living habits, regular work and rest, eat a light diet, avoid spicy and stimulating food, quit smoking and drinking, and take proper exercise to improve body resistance.