The invention provides a method for assessing the power distribution reliability of a large-scale medium-voltage distribution network based on modes. For the large-scale medium-voltage distribution network, the detailed assessment and analysis of power distribution reliability has a large amount of work load and implement difficulty. In the method of the invention, similarities of lines in structural characteristic, electric characteristic, power distribution reliability and other aspects in the distribution networks of the same area are considered, thus the structural characteristic of one set of lines is selected as the basis of classification, and the lines with the similar structural characteristic belong to the same mode, wherein the reliability index of each mode is represented by typical lines therein, thereby greatly reducing the work load and analysis difficulty of reliability assessment. The classification method of the mode is mainly based on the relative factors of the distribution network structure, thus the mode is called as a network reliability characteristic mode. The method is applied to the assessment of the power distribution reliability of the large-scale medium-voltage distribution network.