Left shoulder pain may be due to muscle strain and other factors, or rotator cuff injury, frozen shoulder, cervical spondylosis, coronary heart disease, pleurisy and other reasons. 1. Muscle strain: Athletes, workers and other people move their left shoulder for a long time, which leads to muscle strain and muscle injury of the left shoulder, and thus left shoulder pain. 2. Rotator cuff injury: due to chronic strain and acute injury, resulting in damage to the skin, subcutaneous tissue, joints and ligaments, and thus left shoulder pain. 3. Frozen shoulder: self-limited aseptic inflammation of the left shoulder joint, resulting in pain and impaired movement of the left shoulder. 4. Cervical spondylosis: degenerative lesions and hyperplastic lesions of the cervical joints lead to changes in the local anatomical position, spinal nerve root compression, and left shoulder pain due to involvement of the left dorsal nerve. 5. Coronary heart disease: atherosclerosis of coronary artery leads to myocardial ischemia, resulting in pain in the anterior region of the heart, and radiating pain to the back of the left shoulder. 6. Pleurisy: Infection leads to pleural inflammatory lesions involving the left upper pleura, which irritates the neighboring left shoulder back and leads to left shoulder pain. Left shoulder pain may be related to a variety of factors, and it is recommended to consult a doctor for a definitive diagnosis.