The principle of induced expectoration technique is considered to be related to high osmolarity and hypertonic saline. 1. High osmolality: it may be related to its high osmolality to promote the penetration of water into the airway lumen, causing reflex cough, as well as to promote the shedding of the mucus layer of the airway epithelium, direct stimulation of the airway, accelerated mucus cilia clearance, and direct stimulation of the glands to increase secretion, and so on. 2. Hypertonic saline: inhalation of hypertonic saline can increase bronchial vascular permeability, which in turn causes plasma proteins to flow into the airway lumen, directly stimulating mucus secretion of airway epithelial cells to discharge sputum. The concentration of saline used in the induced expectoration technique <7% will not cause airway inflammation and is relatively safe, with the advantages of being non-invasive, safe, reliable and simple. And hypertonic saline itself can cause bronchial constriction, sore throat, dry throat and other throat irritation symptoms. Therefore, the technique of inducing sputum evacuation must be operated by professionals and requires close attention to changes in the patient’s condition during the induction process.