Abstract: Recently, accompanying with more and more clinical and basic researches, our understanding of evaluating indexes of the diagnosis and treatment in critically ill patients has improved and developed. The treatment of critically ill patients has developed from group treatment and individual treatment to organ-based treatment. Treatments that directly evaluate blood flow of organs and aim at improving blood perfusion and function of organs are increasingly applied in critically ill patients. As an important monitoring and evaluating method for critically ill patients, critical ultrasonography plays an irreplaceable role in the treatment of critically ill patients, and constantly promotes physicians' understanding and evaluating diseases in the critical care. The application of critical ultrasonography revolutionized our understanding of systemic oxygen delivery, the heart, lungs, and other vital organs. At the same time, it promoted the clinical diagnosis and treatment of critically ill patients. How to better apply critical ultrasonography and evaluate organ hemodynamics is a very important problem for physicians of critical care. This article intends to systematically elaborate on the important role and application of critical ultrasonography in the assessment of systemic oxygen delivery and blood supply of organs, so as to provide a reference for clinicians.