The pressure in the pleural cavity is -4 to -6 cmH2O. The pleural cavity is a confined structure enclosed by the dirty pleura and the wall pleura, and when the lungs breathe, a negative pressure is generated in the pleural cavity. The significance of negative pressure in the pleural cavity is that it keeps the lung in an inflated state and facilitates gas exchange. When there is a pneumothorax, the gas in the lung leaks into the pleural cavity, the cavity becomes positive pressure, the lung will be compressed, the effective breathing area of the lung is reduced, and there will be difficulty in breathing and other conditions. If the compressed lung is less than 30%, the gas in the lung can be absorbed by itself after the lung rupture heals. If the gas in the pleural cavity is more than 30%. Patients with severe respiratory distress need to be treated with closed pleural drainage, so that the gas in the pleural cavity can run to the outside world, and the negative pressure in the pleural cavity can be re-formed through the water seal bottle interval, which is conducive to the reopening of the lung.