Ultrasound of pleural fluid has special requirements and requires a sitting position because water flows to the lower part of the chest. When a person is standing, the water is deposited at the lowest point of the chest cavity. In the lying position, the water is spread horizontally throughout the chest cavity and is not easy to be detected; in the sitting position, the water is concentrated in the rib-diaphragm angle or above the diaphragm, which makes it easy to detect the presence of fluid. The detection of pleural fluid requires further clinical management, such as puncture and drainage of pleural effusion, ultrasound for pleural effusion examination, which requires localization with the puncture position. The patient sits backward in a chair, hands grasping the back of the chair, and adopts a thoracic spreading position, which is easy for the physician to maneuver and able to demonstrate the thoracic intercostal space. Ultrasonography is performed on the body surface to mark the location of the effusion, measure the depth of the effusion, and determine the depth of the needle. The puncture point is marked and the needle is inserted through the marked position for puncture treatment. Ultrasound-guided pleural fluid puncture is safer.