The embodiment of the invention discloses a method and a device for determining a power failure range. The method comprises the following steps of: connecting electrical equipment data bases containing electrical equipment nodes and junction points; finding the junction point to which an end point of a power supply point used as a father node belongs; judging whether the junction point contains other end points; if the junction point contains other end points, finding electrical equipment nodes corresponding to other end points according to other end points, wherein the found electrical equipment nodes are used as child nodes of the father node and circularly finding the junction points by using the child nodes as new father nodes; if the junction point does not contain other end points, building a power grid topology model according to the father nodes and the child nodes; and receiving the input power failure electrical equipment nodes, recognizing the nodes below the power failure electrical equipment nodes according to the power grid topology model, and determining a power grid topology containing the power failure electrical equipment nodes and the nodes below the power failure electrical equipment nodes as the power failure range. The power failure range which is determined by the embodiment of the invention is accurate and complete, and the real power failure area can be reflected well.