Medical nutrition therapy has been widely applied in various diseases as a fundamental or even a first-line treatment. Patient who undergoes a disease state especially at the perioperative period can be much improved with the help of nutrition therapy. Precise nutrition diagnosis should be conducted before applying any nutrition therapy. Traditional malnutrition diagnostic process, however, is a two-stage process (nutrition screening and nutrition assessment) which cannot precisely assess nutritional status of surgical patients or the consequences of being malnourished. This article systematically introduced a new nutrition diagnostic process - Three-Stage Diagnosis (nutrition screening, nutrition assessment, and comprehensive evaluation) and discussed its applications during perioperative period.