The reason for coughing up yellow sputum is due to infection of the respiratory tract with purulent bacteria, which causes an inflammatory reaction leading to a large amount of yellow inflammatory exudate. Purulent bacterial infections such as Staphylococcus aureus infection, can cause the lungs, bronchial tubes, tracheal wall secretion of a large amount of purulent secretion, through the bronchial wall of the cilia, in the form of phlegm discharge purulent secretion, purulent secretion contains bacteria, inflammatory cells, necrotic tissue, etc., often manifested as a yellowish pus-colored sputum. Coughing up yellow sputum is common in upper respiratory tract infections, pneumonia, bronchopneumonia, lung abscess and tuberculosis, etc. Bacterial infections are more common. Patients coughing up yellow sputum, it is recommended to go to the hospital in time, complete the relevant laboratory tests to determine the cause of the disease, and follow the doctor’s instructions for treatment.