Parenteral nutrition (PN) refers to the provision of more comprehensive nutrients via routes outside the digestive tract to prevent or correct malnutrition (deficiency), reduce nutritional risk and improve clinical outcomes. At present, PN has been widely used in clinical practice, and academic organizations at home and abroad have released a series of application guidelines. 2008, the Chinese Society of Parenteral and Enteral Nutrition also issued the “Clinical Treatment Guidelines (2008 Edition)”, which put forward relevant guiding recommendations on the rational and standardized application of PN. Therefore, as a part of clinical disease treatment, it is recommended to follow the basic PN application steps and norms so that patients can get good medical services of nutrition treatment and nutrition rehabilitation. I. Nutritional risk screening and nutritional status assessment After patients are admitted to hospital, it must first be clarified which patients need nutritional support. According to the recommendation of the guideline, within 24-48 hours after the patient is admitted to hospital, the nurse or dietitian will screen the patient for nutritional risk or malnutrition; on this basis, a member of the nutritional support team (physician or dietitian) will make further comprehensive assessment of nutritional status, including dietary assessment, anthropometric measurements, laboratory tests and organ function, etc., to determine whether the patient has malnutrition (deficiency) and its degree. Currently, the recommended or commonly used screening methods (tools) are: Nutrition Risk Screening 2002 (NRS-2002), Subjective Global Assessment (SGA) and Mini-Nutrition Assessment (MNA). For those who are at nutritional risk or undernourished, appropriate nutritional support program will be formulated and implemented in time. For patients with nutritional support indications, the first step is to determine whether the function of gastrointestinal tract is sound. When the function of gastrointestinal tract is completely or only partially impaired, enteral nutrition is preferred; if the oral or gastrointestinal intake cannot reach 60% of the target requirement for 5 days or more, the combined application of parenteral nutrition should be considered to supplement the shortage. If the gastrointestinal tract cannot be utilized due to disease or treatment, such as fasting, intestinal failure, intestinal obstruction, intestinal ischemia, active bleeding of gastrointestinal tract, etc., total parenteral nutrition support treatment should be provided. The supply of energy and nitrogen for parenteral nutrition support The supply of energy and nitrogen is the basis of parenteral nutrition support. The amount of energy and nitrogen required depends on the disease, stress, nutritional status, intake and other conditions. For example, for perioperative PN support, energy is currently recommended at 25-30kcal/kg.d (ideal body weight) and protein or amino acid supply at 1.5g/kg.d. The energy ratio of protein:fat:sugar is about 20:30:50%. The energy and nitrogen requirements of patients in different disease states are different, so please refer to the application guidelines for each disease in the Clinical Guideline (2008 edition). The route of parenteral nutrition support can be divided into peripheral venous (PVC) and central venous (CVC) infusion, and central venous placement can be done through peripheral venous puncture with central venous catheter (PICC) or direct percutaneous puncture with central venous placement. The total amount of PN, the osmolarity of the PN mixture, the number of days of PN application, and the venous conditions should be determined. Usually, when the total amount of PN is more than 1500 ml/day, or the osmolarity of PN mixture is ≥900 mosm/L or the duration of PN is more than 2 weeks, and the peripheral venous conditions are poor, PICC or CVC placement can be considered for the infusion of parenteral nutrition mixture. For the total amount of PN less than 1500ml/day, and the osmolarity of PN mixture <900mosm/L, and the application time is shorter than 2 weeks, peripheral intravenous infusion is feasible. In order to achieve the best caloric to nitrogen ratio for parenteral nutrition support, facilitate better absorption and utilization of nutrients, and reduce the occurrence of PN-related metabolic complications, it is recommended to use Total Nutrient Admixture (TNA), i.e. "All In One" (AIO). All In One (AIO) infusion is recommended. Note: In the absence of amino acid solution, glucose solution and fat milk should not be mixed into the "All In One" form (the pH of glucose will affect the stability of fat milk particles). During the period of parenteral nutrition support, clinical and laboratory indicators need to be monitored in order to understand the effect of nutrition support, reduce and avoid complications related to parenteral nutrition, and achieve the best therapeutic effect. Clinical indicators include vital signs (body temperature, blood pressure, pulse rate, respiration), fluid balance (water intake and output, urine volume, various drains, intake), PN infusion response, weight change, etc. In critically ill patients, it is more important to monitor and record the changes of the above indicators. Laboratory tests include routine blood, electrolytes (sodium, potassium, chloride, calcium, phosphorus, magnesium), blood glucose, liver function, kidney function, albumin, prealbumin, lipids, etc. The frequency of tests varies according to the needs of the disease. Indicators such as nitrogen balance, amino acid profile, and immune function can be determined on a case-by-case basis.