Nutritional support is one of the important measures in the treatment of critically ill patients. Reasonable and effective nutritional support can reduce the catabolic reactions of the body under stressful conditions, improve the important organs and immune functions of the body, reduce the incidence of complications, shorten the ICU and hospitalization time, and improve the success rate of rescue of critically ill patients. At present, the proportion of critically ill patients receiving nutritional support in clinical practice is low, and unreasonable nutritional support is still very common. Therefore, how to carry out reasonable nutritional support remains a topic of concern in the treatment of critically ill patients. Under stressful conditions such as severe trauma and infection, the body is in an imbalanced homeostasis, in a high catabolic state, with increased resting energy consumption and disorders of sugar, protein and fat metabolism. If the critical condition persists, the body tissues are continuously consumed, and if timely and sufficient nutritional supplementation is not available at this time, protein depletion of different degrees will occur, affecting the structure and function of organs, which will eventually lead to multi-organ failure, thus affecting the prognosis of patients. On the other hand, due to the serious metabolic disorders and poor tolerance of the body in the early stage of stress in critically ill patients, inappropriate nutrient intake at this time will not only fail to play an effective nutritional role, but also cause metabolic complications, which will also affect the prognosis of patients. Therefore, how to provide appropriate nutritional substrates to maintain cellular and tissue metabolism as well as organ structure and function according to the metabolic status of critically ill patients is an important goal of nutritional support for critically ill patients. In fact, the nutrient requirements of critically ill patients should be based on different disease conditions, different stages of the disease and the function of vital organs of the body. Generally speaking, in the early stage of stress such as severe trauma and infection, the circulatory and respiratory system and internal environment of the body are unstable, so it is necessary to actively treat the primary disease, maintain the stability of the internal environment and the physiological function of important organs and tissues, and start to consider nutritional support only after the circulatory and respiratory system and internal environment become stable. Therefore, in the early stage of critical patients, it is better to supplement simple fluids, electrolytes and 100-200g of glucose, and if nutritional support is needed, the non-protein caloric intake should be controlled at about 20Kcal/(kg・d), and then gradually increased to 25Kcal/(kg・d). Clinical practice shows that in the early stage of stress in critically ill patients, restrictive caloric intake can help the body to smoothly pass through the traumatic stress stage and reduce metabolic disorders, and as the condition stabilizes, caloric and nitrogen intake should be gradually increased. Carbohydrate is the main energy supplying substance, which should account for about 60% to 75% of the total non-protein calories. It is worth noting that stress hyperglycemia is a common problem in critically ill patients, which is closely related to the restricted glucose oxidation, enhanced glucose xenobiotic and insulin resistance in peripheral tissues during severe stress, when excessive glucose intake will increase the existing hyperglycemia, aggravate the metabolic disorder and organ function damage, thus affecting the patient’s prognosis. Therefore, when administering parenteral nutrition to critically ill patients, excessive glucose intake should be avoided, and the glucose infusion rate should be controlled at 2-2.5 mg/kg.min, while insulin should be applied to control blood glucose. Current evidence suggests that strict control of blood glucose levels (6.1-8.3 mmol/L) can significantly improve the prognosis of critically ill patients. Fat emulsion is an important nutritional substrate in parenteral nutrition, and its main physiological functions are to provide energy, constitute body tissues, supply essential fatty acids and carry fat-soluble vitamins. About 25% to 40% of total non-protein calories in critically ill patients are provided by fat emulsions (except in patients with severe hyperlipidemia). Because of the high content of linoleic acid and low content of antioxidants in traditional long-chain fat emulsions derived from soybean oil, the proliferation and activity of lymphocytes, monocytes and neutrophils can be inhibited in high metabolic states such as trauma and infection, and the production of pro-inflammatory cytokines such as TNF-a and IL-1b can be increased, leading to impaired immune function, increased lipid peroxidation production, and an inflammatory regulatory response. Therefore, long-chain fat emulsions of pure soybean oil origin should be used with caution in critically ill patients. Clinical studies have shown that structural fat emulsions, fat emulsions containing olive oil or fish oil that have emerged in recent years are superior to traditional long-chain fat emulsions of soybean oil origin in metabolism, nitrogen conservation, prevention of oxidative stress, down-regulation of inflammatory responses and maintenance of organ function, and are therefore more desirable energy substances for critically ill patients. Clinically, the appropriate daily amount of fat emulsion is 1 to 1.5 g/kg, and the maximum amount should not exceed 2 g/kg. Appropriate protein supplementation can play a role in correcting negative nitrogen balance, repairing damaged tissues, and synthesizing proteins. Clinically, the daily nitrogen intake of surgical critical patients should be about 0.15-0.2g/kg.d. Excessive nitrogen intake does not improve the nitrogen saving effect, but rather increases the metabolic load of the body. Amino acid solution is currently the main form of protein supply in clinical practice, and the choice of an ideally formulated amino acid solution can achieve better nutritional support purposes. Generally speaking, balanced amino acid solutions can meet the nitrogen needs of most critically ill patients.