Left back pain with mild dyspnea on deep inhalation is usually considered to be caused by left-sided lung diseases such as lung infection, tuberculosis, lung cancer, pleurisy, etc., or left posterior rib fracture and other diseases. Generally, a clear diagnosis is needed before further treatment or management. 1. Left-sided lung diseases such as lung infection, tuberculosis, lung cancer, pleurisy, etc. may invade the pleura and cause pleural inflammation, which will aggravate the inflammatory pleura rubbing against each other during deep inhalation and cause left back pain, and at the same time may be accompanied by slight dyspnea due to lung inflammation. 2. Left posterior rib fracture: deep breathing can cause mild displacement of the broken end of the rib, resulting in left posterior back pain, and the patient may suffer from back pain, breath-holding, and phlegm, accompanied by mild dyspnea. Left posterior back pain with mild dyspnea during deep inhalation is recommended for further counseling or treatment in the hospital.