Hemoptysis is defined as bleeding from the respiratory tract below the throat through the oral cavity. According to the amount of hemoptysis, it can be divided into blood in sputum and hemoptysis with large mouth. The site of hemorrhage is mostly confined to the bronchi and lungs. There are many causes of hemoptysis, which are mainly divided into the following four categories of diseases. 1. Diseases of pulmonary origin: such as tuberculosis, pneumonia, lung abscess, pulmonary cyst (congenital or acquired), malignant tumors, etc.; 2. Bronchial diseases: such as chronic bronchitis, bronchiectasis, endobronchial tuberculosis, bronchial cancer (primary lung cancer), benign bronchial tumors, bronchial foreign bodies, etc.; 3. Cardiac and pulmonary vascular diseases: such as pulmonary stasis (chronic cardiac insufficiency, mitral stenosis), pulmonary arteriovenous fistula, pulmonary hypertension, pulmonary thromboembolism, etc.; 4. Systemic diseases and other causes: such as acute infectious diseases, hematological diseases (e.g. thrombocytopenic purpura, leukemia, etc.), connective tissue diseases (systemic lupus erythematosus, rheumatoid arthritis, dry syndrome, etc.), systemic vasculitis (Wegener’s granulomatosis, microscopic vasculitis, etc.), etc. To distinguish the etiology of hemoptysis, past medical history is important. If recurrent hemoptysis is present, attention should be paid to the presence of pulmonary or cardiogenic disease. In young adults, more attention should be paid to tuberculosis, bronchiectasis, and pulmonary cysts. In middle-aged and older hemoptysis with chronic cough, as well as smokers should be alerted to bronchopulmonary cancer. In addition to the above preliminary judgment, patients with hemoptysis should also undergo appropriate examinations, such as chest X-ray (or even chest CT), blood routine, coagulation function, autoimmune antibody, cardiac ultrasound, fiberoptic bronchoscopy, etc., to help determine the cause of hemoptysis. Although hemoptysis of cardiac and pulmonary origin is common, some hemoptysis is more difficult to diagnose. In conclusion, hemoptysis is a very important clinical sign and should be taken seriously, regardless of whether the amount of hemoptysis is large or small. Due to the complex etiology of hemoptysis, it is difficult for patients to determine the cause on their own, so they should seek medical attention in a timely manner to avoid delaying treatment.