The causes of left chest and left back pain for more than three months include left chest soft tissue contusion, chest disease, heart disease and so on. 1. Chest soft tissue contusion: such as the left side of the chest rib joint muscle strain, chest soft tissue trauma, etc., will cause the left chest pain, and can lead to back pain radiating. 2. Chest diseases: such as pleural inflammation, lobar pneumonia, pneumothorax, pleural effusion, lung tumors and so on. 3. Heart diseases: such as myocardial ischemia, coronary heart disease, angina pectoris, myocardial infarction, etc., which can cause chest pain and may radiate to the lower jaw, neck, and the upper left side of the back. If the above symptoms occur, it is recommended to go to the hospital to complete the relevant examinations to identify the cause of the disease and under the guidance of the doctor to carry out regular treatment in a timely manner.