Due to frequent participation in interdepartmental consultations, patients are mostly critically ill, and it has been found that nutritional support therapy is often absent or insufficient in the treatment measures for these critically ill patients. In recent years, nutritional support for critically ill patients has received increasing attention. Malnutrition leads to increased morbidity and mortality, delays patient recovery; increases the incidence of complications and prolongs hospitalization; malnutrition occurs in up to 80% of hospitalized patients. With the continuous development of clinical nutrition and the advancement of application technology, nutrition therapy has been widely used in clinical practice. Therefore, each clinician should pay attention to the nutritional supplementation of critically ill patients. Causes of malnutrition: According to statistics, the incidence of malnutrition in hospitalized patients is 30-80%, which can occur in any clinical department. Causes of malnutrition include reduced nutritional intake, increased nutritional requirements and impaired nutrient absorption or utilization. Clinically, it is common in trauma, infection, shock, burns, stress and persistent hyperthermia causing the release of stress hormones, inflammatory mediators and cytokines resulting in high catabolism, impaired glucose utilization, ketone body production and lean body mass (LBM) breakdown. Patients like those who cannot eat in abdominal surgery, tumor patients after major surgery, patients with severe compound injuries, multiple limb fractures, patients with stroke and severe cranio-cerebral injury or co-infection, patients with severe lung infections, patients with application of ventilator-assisted ventilation, and patients with acute respiratory distress syndrome (ARDS) have a 100% incidence of malnutrition. Patients with renal transplantation, acute phase of renal disease, patients requiring hemodialysis, as well as patients with rheumatic immune diseases and hematologic diseases can have different degrees of malnutrition. The hazards of malnutrition include immune dysfunction, increased tissue decomposition, muscle weakness in various parts of the body, cardiac and pulmonary dysfunction, changes in the epithelial structure of the small intestine and destruction of the mucosal barrier resulting in translocation of the flora, impaired absorption of nutrients, emotional indifference or depression of the patient, resulting in complications of the corresponding diseases such as persistent hyperthermia, non-healing fractures, chronic osteomyelitis, abdominal abscesses, anastomotic fistula, delaying the patient’s recovery This can lead to complications such as persistent hyperthermia, fracture healing, chronic osteomyelitis, abdominal abscess, anastomotic fistula, delayed recovery time, increased hospitalization and medical costs, and even increased mortality. Nutritional support route and choice: enteral nutrition (EN) is preferred, but parenteral nutrition (PN) is chosen when gastrointestinal function is unsafe. Both have their advantages and disadvantages, and sometimes a combination of both is needed. Nutritional guidelines of the Chinese Medical Association’s Critical Care Branch guide; critically ill patients are often combined with metabolic disorders and malnutrition and need to be given nutritional support; nutritional support for critically ill patients should be started as early as possible, and as long as the anatomy and function of the gastrointestinal tract allow and can be used safely, enteral nutrition should be started as early as possible when conditions allow; for any reason that the gastrointestinal tract cannot be used or applied insufficiently, parenteral nutrition or combined application of enteral nutrition should be considered. In recent years, a series of related studies have shown that certain nutrients have immunopharmacological effects, and the addition of these specific nutrients, such as glutamine, arginine, ω-3 fatty acids (or fish oil), nucleosides and nucleotides, can not only prevent and treat nutritional deficiencies, but also stimulate immune cells in a specific way to enhance response function, maintain normal and moderate immune responses, regulate cytokine production and release, reduce harmful or excessive inflammatory responses, and maintain intestinal barrier function, a new concept called nutritional pharmacology or immunonutrition.