Patients with severe acute kidney injury (AKI) often need renal replacement therapy (RRT）with a high morbidity and mortality. For patients with chronic renal failure, the aim of blood purification is renal replacement; but for patients with AKI, although customarily called RRT, the aim of blood purification is not “renal replacement”, but extracorporeal “renal support and protection”, that is, supporting and protecting temporally failed kidney, removing damage factors, avoiding renal reinjury and looking forward to restore renal function. This article provides a detailed explanation of the differences between renal replacement and renal support from the perspective of organ protection, as well as the key links of RRT and extracorporeal multiple organ support for patients with severe AKI.