The incidence of moderate to severe malnutrition in malignant tumors is very high. The latest research shows that the incidence of moderate to severe malnutrition in Chinese tumor patients is 58%. Malnutrition significantly affects the clinical outcome of tumor patients, and nearly 25% of tumor patients die directly from malnutrition. Many malnourished tumor patients often show high resting energy expenditure and hypermetabolism. The factors causing these changes mainly include two aspects: ① tumor load; ② the related factors caused by tumor, including inflammation, neuroendocrine disorder, radiotherapy and chemotherapy, intestinal microecological disorder, loss of appetite and intake, etc. These factors are mixed together and activate the increased energy consumption of different tissue and organ systems, enhanced catabolism and decreased anabolism through complex mechanisms that are not fully clarified, resulting in malnutrition in tumor patients. Because the factors and mechanisms causing malnutrition (including cachexia) in tumor patients are complex and diverse, it is necessary to comprehensively evaluate and diagnose malnutrition in tumor patients first, and then carry out effective comprehensive nutritional intervention and drug intervention to