There are many reasons for the pain in the left shoulder joint, the common ones are traumatic injury, frozen shoulder, cervical spondylosis, myocardial infarction and other diseases. 1. Traumatic injury and strain: due to traumatic impingement or long-term strain, it will lead to the damage of local soft tissues of the shoulder joint, resulting in the accumulation of lactic acid in the muscle or aseptic inflammation, and the symptoms of pain in the left shoulder joint can appear. 2. Frozen shoulder: It is a self-limiting aseptic inflammation of soft tissues around the shoulder joint for unknown reasons, and most of them are thought to be the adhesion and stiffness of the tissues around the shoulder joint due to the joint action of synovitis and joint capsule fibrosis, which can be manifested as pain in the shoulder joints and around the joints, and is often accompanied with the restriction of the joint activities. 3. Cervical spondylosis: due to long-term strain on the cervical vertebrae, osteophytes or prolapsed intervertebral discs, the protruding intervertebral discs compress the spinal cord, nerve roots and vertebral arteries, etc., which may result in neck pain, pain in the left shoulder crease and radiating pain in the left side of the upper limbs, and in severe cases, upper limbs may be heavy and weak with numbness and other symptoms. 4. Myocardial infarction: due to coronary artery obstruction, myocardial ischemia caused by blood supply, manifested as precordial pain or suffocation, pain is mainly in the back of the sternum, but also can be upward radiation to the left shoulder, back, and the left upper extremity and other parts of the body, often accompanied by changes in blood pressure, heart rate. The left shoulder pain may have other causes, should go to the hospital in a timely manner, improve the examination to clarify the cause of the disease, under the guidance of the doctor standardized treatment.