The present disclosure relates to a flexible display substrate and a flexible display device. The flexible display substrate includes a flexible base substrate and a pixel layer. The flexible base substrate includes: a plurality of opening regions, as well as a plurality of island regions and a plurality of bridge regions divided by the plurality of opening regions; and a pixel layer including a plurality of display units arranged in the plurality of island regions respectively, in which each of the plurality of display units is provided with a first groove, the first groove divides the corresponding display units into a display section and a dam section, and at least a part of the dam section is located between the display section and the bridge region adjacent to the display section.