The causes of phlegm include infection, tumor, left heart failure, etc. It is recommended that patients seek medical treatment in time. 1. Infection: such as chronic bronchitis, bronchial dilatation, pneumonia, chronic obstructive pulmonary disease, etc., due to inflammation stimulation leads to the increase of inflammatory exudate in trachea and bronchial tube, which triggers sputum, and the patient may also have fever, cough, chest pain and other adverse reactions. 2. Tumor: such as lung cancer, its tissue necrosis or exudate caused by local inflammatory reaction can make the patients have the symptom of excessive sputum, and the patients can also be accompanied by coughing, hemoptysis, wheezing, chest tightness, shortness of breath, emaciation, chest pain and other uncomfortable symptoms. 3. Left heart failure: patients with left heart failure have reduced ejection capacity of the heart, pulmonary stasis, increased exudate in the alveoli, which triggers phlegm, and patients may also have different degrees of dyspnea. If the symptoms of sputum are not relieved or accompanied by other discomforts, the patient should consult a doctor in time to find out the cause of the disease under the guidance of the doctor and carry out the appropriate treatment.