Causes of blood in stool in late stage of lung cancer usually include massive hemoptysis, metastasis of cancer cells in gastrointestinal tract, etc., which need timely medical treatment once discovered. 1. Clinical symptoms of lung cancer can be cough, sputum, blood in sputum or hemoptysis, etc. When the amount of hemoptysis increases to a certain extent, the blood swallowed into gastrointestinal tract will be discharged along with stools, which will lead to the symptom of blood in stool. 2. When lung cancer metastasis occurs in gastrointestinal tract, due to dry and hard stool, the metastatic gastrointestinal tumor will easily bleed after friction with stool, especially the tumor metastasized to the low colorectal area is more prone to bleed, among which the bleeding of metastatic cancer in the left half of colon is the most common, and the amount of bleed is also relatively large.