The patient has a high body temperature with flaccid fever. The white blood cell count is elevated, with neutrophils increasing to more than 80% and the nuclei shifting to the left. Chest X-ray is the main diagnostic method for septic chest. The free pleural fluid is first deposited at the base of the thoracic cavity, usually between the base of the lung and the diaphragm, causing the lung tissue to float slightly upward. In case of small amount of effusion, the angle of rib diaphragm is blunted and the amount is about 200ml. If the patient cannot take chest X-ray in sitting or standing position for some reasons, attention should be paid to comparing the density on both sides of the lying chest X-ray, and the density on the side of the effusion is generally increased, and the lateral lying horizontal projection of the affected side can also be used. A small amount of effusion can be shown on the lateral wall of the affected thoracic cavity, and there is a layer of uniform deepening shadow between the inner edge of the rib cage and the outer edge of the lung. In the case of moderate amount of fluid accumulation, the X-ray shows an arc-shaped dense fluid shadow in the lower chest with high external and low internal levels, the shadow covers the entire diaphragm surface, and the volume of fluid accumulation is about 500-1000 ml. In the case of large amount of fluid accumulation, the fluid reaches the lung tip, the lung tissue is compressed and atrophied, the translucency on the affected side is further reduced, the volume of the thoracic cavity increases, the rib space widens, the rib position flattens, the mediastinum shifts to the healthy side, the diaphragm decreases, and on the left side the contrast is easy to show due to the air in the gastric alveoli. In the right side, the liver is not easily distinguished because of the similar density of the fluid. When the effusion is combined with pulmonary atelectasis, the changes of the mediastinum, diaphragm and thorax are often not obvious, and the image of the effusion with high external and low internal level varies with the location of pulmonary atelectasis, which is mostly atypical. In combination with pneumothorax or bronchopleural fistula, the liquid-air surface can be seen. The localized septic chest is mostly seen in the posterior and lateral walls of the chest cavity, and the localized hyperdense shadow can be seen on X-ray, with deeper density in the central part and shallower density around it. It is often differentiated from pleural lesions, lung tumors, subdiaphragmatic abscesses, and liver abscesses. It is often distinguished from pleural lesions, lung tumors, subphrenic abscesses, and liver abscesses. Interlobular effusion is a pleural effusion located in the interlobular fissure, which must be viewed in multiple directions under fluoroscopy in order to show the edge of the septic shadow when the X-ray is in the same direction as the interlobular fissure, mostly with clear edges, uniform density, shuttle-shaped, with the long axis of the shadow accumulating at both ends in the same direction as the interlobular fissure, and may be rounded when there is a lot of fluid. The x-ray of pneumopericardial effusion shows the highest point of the diaphragm shifted outward on the posterior anterior film and shifted backward on the lateral film, or the thickened mechanic’s diaphragm shadow is seen. When a shadow resembling the elevated diaphragm is found, a fundic fluid is suspected, and the true diaphragmatic position can be shown after the fluid flows away from the diaphragm using horizontal projection in the prone position or on the affected side. CT examination The septic chest shows a bow-shaped uniform dense shadow parallel to the chest wall, and changing the position can determine whether the fluid can move. A large amount of effusion enters the lung fissure, which can compress and shift the lower lung inward and backward. A large amount of effusion is adjacent to the posterior margin of the right lobe of the liver, and CT scan shows a blurred posterior margin of the right lobe of the liver with indistinguishable boundaries. This is a characteristic change of pleural effusion and is called “interfacial sign”. Ultrasound In the early stage, when there is no fibrin deposition to form pleural hypertrophy, there is no sediment in the fluid, and the dark area of fluid is clear with no light spot inside. When there is a large amount of fluid accumulation, the lung tissue is compressed and the gas in the lung is absorbed, and ultrasound can see a triangular dense shadow in a large liquid dark area that floats with breathing. When the probe is close to the diaphragm, a diaphragmatic shadow with a circular light band can be seen, and the latter forms a wedge-shaped angle with the chest wall, i.e. the rib-diaphragm angle. The final and definitive diagnosis can be made by aspiration of pus by thoracentesis. The appearance, character, color and odor of the pus are helpful in determining the type of pathogenic bacteria. Bacterial culture and drug sensitivity tests help to select effective antibiotics.