In the condition of strong magnetic storm, geo-magnetically induced current(GIC) flowing through the neutral point of the transformer, causes the increase of reactive power loss of the transformer. This might lead to overload of the reactive power compensation device, drop of the bus voltage and occurrence of a chain fault, which in turn causes a blackout of the system. Comparing the characteristics of each link in the fault chain and the development law of the power system blackout caused by magnetic storms, the fault chain model is used to simulate the process of power outage under conditions of geomagnetic storm. The article determines the propagation path of cascading failures based on the self-organizing critical theory and the safety margin of non-faulty circuits. Combining the IEEE-RTS 79 system parameters, the geo-location of each bus is estimated. Taking this system as an example and using Power World simulator, the research results verify that the proposed model can reflect the geomagnetic storm parameters against the power system fault chains and the identification of weak links under given grid conditions. The research results can provide a basis for quantifying and preventing disasters in the magnetic storm condition.