The common reasons for spitting thick black sputum include smoking, working environment, respiratory system diseases such as lung cancer, digestive system diseases such as esophageal cancer, etc. 1. Smoking: Cigarettes contain tar and other substances, long-term smokers, the toxic substances in the smoke stimulate the inflammation of the mucous membrane of the respiratory tract, and can spit thick black sputum. 2. Working environment: If there is soot in the long-term working environment, such as coal workers, inhaling a large amount of dust and mixing it with secretion in the trachea, they will also spit thick black sputum. 3. Diseases of the respiratory system: such as lung cancer, lung abscess or tuberculosis, blood in the sputum exists for a long time, black sputum may appear. 4. Diseases of digestive system: such as advanced esophageal cancer, tumor cells invade into lungs, causing black sputum. There may be other reasons for spitting thick black sputum, so it is recommended to go to the hospital in time, improve the examination, clarify the cause of the disease, and carry out standardized treatment under the guidance of doctors.