Most of the sputum with fishy smell is caused by respiratory tract bacterial infections, in addition, respiratory malignant tumors, purulent sinusitis can also appear sputum with fishy smell phenomenon. 1. Respiratory tract infection: clinically, the bacteria that cause respiratory tract infection mainly include gram-negative anaerobic bacteria, gram-positive anaerobic bacteria, Pseudomonas aeruginosa, Staphylococcus aureus, Klebsiella and so on. When bacterial infection occurs in the respiratory tract, leading to pneumonia, lung abscess, bronchial dilatation co-infection, there can be a large amount of sputum with a fishy odor. 2. Malignant tumor of respiratory system: such as lung malignant tumor or nasopharyngeal carcinoma, tumor necrotic tissue cells shedding liquefaction, retention in nasopharyngeal or airway memory for a long time, sputum will also appear fishy smell. 3. Purulent sinusitis, a large number of pus cells and necrotic blood cells mixed, can reflux to the pharynx and cough out, can also appear sputum fishy odor. Therefore, when the sputum appears fishy odor, according to the specific symptoms, combined with the medical history and related examinations, such as routine blood, chest radiographs or chest CT, sputum bacterial culture, etc., a clear diagnosis, targeted treatment.