Abstract: Reactive power loss of the transformer increases during the geomagnetic induced current (GIC) flowing through the neutral point of the transformer. As a result, the reactive power compensation device is overloaded, the bus voltage drops, and a chain fault occurs in the system, causing a blackout. It is not realistic from the perspective of economy and technology that the measures to suppress GIC at each substation are to be applied. Based on this, this paper first analyzes the change rule of reactive power loss of substations under different magnetic storm intensity and direction and then defines the substation reactive power loss severity index. The indicator itself only averages the actual reactive power losses, and does not change the relative relationship between the magnitudes of reactive losses between transformers. According to this indicator, the identification method for the areas susceptible to magnetic storms in a given system is proposed, which can provide a basis for grid planning and operation of system dispatchers under the influence of geomagnetic storms. Finally, Xinjiang 750kV power grid is taken as an example to verify the feasibility of the proposed algorithm and the vulnerable areas in the example which are pointed out.