Blood in stool in the late stage of lung cancer is mainly considered for two reasons: 1, it may be that lung cancer patients have a large amount of hemoptysis, which is discharged through stool after swallowing; 2, intestinal metastasis of lung cancer occurs directly, and tumors with intestinal metastasis are prone to bleed easily after friction with stool, especially tumors metastasized to the low colorectal part are more prone to bleed, because the stool is dry and hard, which makes it more prone to bleed by friction with the intestinal mucous membrane, and bleeding of metastatic cancer of the left half of the colon is more frequent and it is also more common. In right half colon metastatic cancer, the stools are often in the form of semi-fluid juice, so the amount of bleeding is relatively less, plus it is mixed in stools with color change, which is often not easy to be detected.