The causes of pain below the left shoulder are commonly due to pectoral muscle injury, cervical spondylosis, deltoid disease, etc. The details are as follows: 1. Pectoral muscle injury: The front and bottom of the left shoulder is covered by pectoral muscle, if muscle injury occurs, it will manifest as pain symptoms below the left shoulder, and can be accompanied by limited shoulder joint activities, and pain is obvious when pressing. 2. Cervical spondylosis: Neurogenic cervical spondylosis can cause radiating pain symptoms below the left shoulder, mainly related to nerve compression. It is mainly related to nerve compression, and can be detected by MRI of cervical spine, and the severity of the lesion can be observed. 3. Deltoid disease: such as deltoid strain and inflammation of the bursa in the deep side of the deltoid muscle can lead to pain symptoms below the left shoulder, and can be accompanied by weakness of shoulder joint activities, which can be relieved by active treatment.