Pain on the left side of the chest may be caused by angina pectoris, pneumothorax, pleurisy, and other causes such as myocardial infarction. 1. Angina pectoris: angina pectoris is mainly caused by myocardial hypoxia due to the decrease of myocardial blood flow, and the patient will experience pressure pain in the retrosternal and precordial regions. As a result, patients will have pain on the left side of the chest. 2. Pneumothorax: Pneumothorax is considered to be caused by chest trauma, pulmonary herpes, chronic obstructive emphysema and other reasons. Patients mainly manifest chest pain, dyspnea, chest tightness, etc. If patients have pneumothorax on the left side of the chest, they will have the symptom of pain on the left side of the chest. 3. Pleurisy: Pleurisy can be caused by autoimmune diseases, bacterial or viral infections, malignant tumors and other factors. Patients will have chest pain, cough and sputum, dyspnea, chest tightness and other symptoms, so patients can manifest as pain on the left side of the chest. 4. Other: pain on the left side of the chest may be caused by myocardial infarction, costochondritis, lung cancer, aortic coarctation, reflux esophagitis, mastitis, trauma and other diseases. It is recommended that patients with pain on the left side of the chest need to consult a doctor in a timely manner, and the doctor will choose the appropriate measures according to the cause of the treatment.