The field development practices at extra high water-cut stage demonstrated that the oil recovery can still be enhanced a lot by the following water-flooding even if the water-flood mode is not changed and the areal and vertical sweep efficiency are very high.This phenomenon can not be interpreted by the conventional theory recovery definition in which the theory recovery is defined as the product of displacement efficiency, areal sweep efficiency and vertical sweep efficiency. Moreover, it is difficult to theoretically give advice for further improving oil recovery at extra high water-cut stage based on the conventional comprehension and the definition of the recovery. The laboratory experiments proved that the sweep degree of microscopic pores, which are in the areal and vertical sweep scope, increased continuously besides the increment of the macroscopic areal and vertical sweep efficiency during the water-flood process. On one hand, the number of the swept pores increased continuously, on the other hand, the swept volume (swept scope) of the swept pores became greater too. The conventional definition of theory recovery can only characterize the relevance of areal and vertical sweep efficiency to recovery, and not sufficient to represent the sweep features of the rock pores. Therefore, the definition of pore sweep extent is presented, and then, it is used to deduce a new evaluation expression of the water-flooding recovery. It indicates that the increment of the pore sweep extent is the main factor for improving oil recovery at extra high water-cut stage.