A chest X-ray image after trauma reveals fluid accumulation in the pleural cavity and the thoracic cavity is punctured with blood, which is called traumatic hemothorax. According to statistics, in all chest trauma, 70% of patients have different degrees of hemothorax. When the chest wall injury is connected with the pleural cavity or there is injury to the intra-thoracic organs, all of them can produce hemothorax or hemopneumothorax. Corresponding to traumatic hemothorax is spontaneous hemothorax, which is the spontaneous appearance of intrathoracic hemorrhage without obvious trauma, such as lung cancer rupture. Traumatic hemothorax can be divided into small hemothorax, medium hemothorax and large hemothorax according to the amount of blood in the chest cavity. A small amount of hemothorax is defined as less than 500 ml of blood, when the chest X-ray shows that the angle of the rib diaphragm disappears or the fluid surface of the hemopneumothorax does not exceed the top of the diaphragm. When the accumulated blood volume is 500-1500ml, it is a medium hemothorax, and the upper boundary of the fluid surface on the chest X-ray reaches the plane of the pulmonary hilum. In massive hemothorax, the amount of accumulated blood in the chest cavity exceeds 1500 ml, the lung is severely compressed and atrophied, and the chest film shows that the fluid surface reaches the upper lung field. The patient did not have symptoms of intrathoracic hemorrhage after the trauma, and physical examination and laboratory tests did not reveal the features of fluid accumulation in the chest. However, a few days after the injury, the patient felt some discomfort and suffocation in the chest, but the chest radiograph confirmed the presence of a hemothorax or even the presence of a large amount of hemothorax. The reason for this situation may be a rib fracture, which did not bleed at that time, and later, when the body moved, the fracture severed end was displaced and pierced the intercostal vessels, or the vessels had a rupture but were temporarily closed by a clot after the injury, and later the clot was dislodged and other reasons, resulting in the presence of blood accumulation in the thorax several days after the injury. Therefore, repeat chest X-ray should be performed within a short period of time after chest trauma to indeed prove that there is no obvious injury or bleeding in the thoracic cavity. In fact, the severity of delayed hemothorax is not fatal and the treatment is no more special; the key is to avoid negligence, otherwise it affects the timely treatment of the patient.