Abstract: Small-for-size syndrome is a common clinical syndrome after liver transplantation and hepatectomy, which is common in postoperative patients with insufficient residual liver volume to maintain normal function. In recent years, several studies have found that progression of small-for-size syndrome is not determined only by the “size” of the liver graft or remnant, but by the hemodynamic parameters of the postoperative hepatic circulation, especially portal vein flow. The liver dynamic watershed theory analogizes the hepatic blood flow to a river. Compared with the traditional Couinaud segmentation, the dynamic watershed theory pays more attention to the dynamic changes of hepatic blood flow. This paper provides a new perspective to understand the etiology and prevent small-for-size syndrome.