Gastroscopy cannot exclude 100% of gastric cancer. Gastroscopy is an effective way to detect early gastric cancer patients. Through gastroscopy, we can clearly see the morphology of gastric mucosa, as well as the location, size and shape of lesions, and do gastric tissue biopsy to make a clear diagnosis through pathological examination. However, for some types of gastric cancer, especially the very early ones with small lesions, such as mucinous adenocarcinoma and indolent cell carcinoma, which are mainly infiltrated under the mucosa, the mucosal layer of the lesion area is often intact, but the cancer cells mainly infiltrate along the submucosa layer, so doctors who are not experienced enough in gastroscopy may not be able to detect the abnormalities of gastric mucosa and sometimes miss the diagnosis. In addition, there are some small cancerous lesions of gastric ulcer, less than 1 cm in diameter, and the mucosa of the cancerous lesion will appear pseudo-healing after taking oral acid inhibiting drugs, but the cancer cells are still growing rapidly and can cause metastasis. For this kind of gastric cancer, it is risky to do gastroscopy alone to exclude gastric cancer. In conclusion, gastroscopy can rule out most gastric cancers, but some rare cases may be missed. Therefore, it needs to be combined with other examination methods to make a comprehensive judgment.