The invention discloses a third defense line configuration method adaptive to the scale to an isolated power grid, and belongs to the technical field of electric power system automation. A high-frequency generator tripping scheme adopts two-stage configuration, the first stage is used for inhibiting a frequency rise of the whole network, and the second stage is used for preventing overspeed protection action of a unit, and sets a second stage action constant value on the basis of an overspeed protection action constant value of the unit; and a configuration scheme of the first stage of high-frequency generator tripping and low-frequency low-voltage load shedding includes a quick action area and a compensation area, and according to an installing characteristic and a load characteristic of the isolated power grid, a principle of maximizing an action constant value range of the last round of the quick action area and the first round of the compensation area is used to arrange action constant values of other rounds. The third defense line configuration method adaptive to the scale of the isolated power grid can give full play to regulating characteristics of the unit and load in the isolated power grid, and can effectively avoid the problem of overcontrol caused by multi-round successive action due to a too small round range in a traditional configuration method.