The invention relates to a method and a system for identifying key branches and dominant stability modes of a power grid based on a response. The method comprises the following steps of according to active power and change conditions of branches, obtaining branch power mode indexes and dominant instability mode identification indexes of the branches; when the power mode index of a certain branch is greater than a preset value, judging a stability situation of the power grid according to a change situation of the power mode index of the branch; and judging the dominant stability modes accordingto the dominant instability mode identification indexes. According to the technical scheme, the power mode indexes of the branches are constructed to identify the key branches; the dominant instability mode identification indexes are constructed; wide-area measurement information of the power grid is used for performing calculation to identify the dominant stability modes; and the method and thesystem have important significance for relevant research of dominant stability mode identification, wide-area coordination control and the like of the large power grid.