Pleural effusion is classified as small, moderate and large under ultrasound. A small amount of pleural effusion is located at the base of the thoracic cavity and appears as a long echogenic zone between the base of the lung and the diaphragm, or as an echogenic zone at the angle of the rib-diaphragm, and its shape and width vary with the respiratory position. In a moderate amount of pleural effusion, the upper boundary of the liquid dark zone generally does not exceed the level of the sixth posterior rib, and the pleural fluid extends upward beyond the rib-diaphragm angle, compressing the lower lobe of the lung, and the liquid dark zone increases in extent and widens in depth. A large amount of pleural effusion means that the upper boundary of the liquid dark area exceeds the level of the sixth posterior rib, the lung is partially or completely atrophied in the direction of the mediastinum, the volume of the lung becomes smaller, the diaphragm shifts downward, the diaphragmatic echogenic band flattens, the heart shifts to the healthy side, and most of the thoracic cavity is a liquid anechoic area, and changes in breathing and body position have little effect on the pleural effusion.